Earth Systems Science · Module 2

Where does the rain on this campus end up?

A storm that once soaked into this land now reaches the bayou in hours, running brown with mud.

That mud began as solid rock. Follow it, and the rain, from ridge to bayou.

Start → Jump to my project Formula sheet
Rain on a campus: where it goes A cross-section, not to scale. Rain falls on a campus between a rocky ridge and a bayou. On the paved lot it runs off to a storm drain and a pipe that empties into the bayou within hours, and the bayou water carries mud. Under the grass, some rain soaks in to the water table and seeps toward the bayou as groundwater over days to millennia. On the ridge, rock is breaking into grains. An inset bar chart shows the share of rain that runs off: about 10 percent on natural ground cover and about 55 percent where 75 to 100 percent of the ground is paved. rain Rain that runs off natural cover 10% 75–100% paved 55% runs off in hours soaks in water table groundwater seeps: days to millennia rock breaks into grains bayou muddy water not to scale
Where 75–100% of the ground is paved, about 55% of rain runs off. On natural ground, about 10% does.

Start here · The big picture

Follow one grain, and the rain that carries it

Four parts of Module 2, in the order a grain travels Four panels joined by arrows, from bedrock on a ridge to the bottom of the bayou. Part 1, Force Sculptors: ice and force break the rock. Part 2, Rock Rot: water changes the minerals and makes soil. Part 3, Sediment Trail: runoff and streams carry the grains and drop them. Part 4, Soak and Slide: rain that soaks in becomes groundwater, and soaked slopes can slide. Break Part 1 Force Sculptors Rot Part 2 Rock Rot Carry Part 3 Sediment Trail Soak Part 4 Soak and Slide bedrock on the ridge the bottom of the bayou

Four parts, one driving question.

Your team leads one part and teaches it. The class needs all four to follow a grain, and the rain, from ridge to bayou.

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Part 1 · Force Sculptors lead this part

Breaking rock: force, wind, and ice

  • Tell weathering from erosion.
  • Explain why freezing water splits rock.
  • Calculate how much surface broken rock exposes.
  • Explain how wind sorts sand from dust.
  • Tell glacial till from outwash.
Ice splits a rock while wind blows sand away A rock block split by a crack. Ice crystals in the crack push the two sides apart. To the right, wind blows over the ground: sand hops downwind while larger gravel stays behind. wind

Part 1 · Key idea

Weathering breaks rock in place; erosion carries it away

Weathering versus erosion Two panels. Left, weathering: a boulder is cracked and crumbling, but it sits in the same spot it started in; nothing has moved. Right, erosion: a stream on a slope carries broken pieces downhill, away from where they formed. Weathering breaks or changes rock Erosion moves the pieces same spot, years later nothing has moved pieces carried downhill by water, wind, ice, gravity

Weathering breaks or changes rock where it sits. Erosion is the moving step: water, wind, ice, or gravity carries the pieces off.

One can happen without the other.

Say it every timeWeathering stays in place. Erosion moves it.

Part 1 · Key idea · Lesson: Mechanical weathering

Five ways force breaks rock without changing it

Five kinds of mechanical weathering Five panels. Frost wedging: water freezes in a crack and pushes it open. Unloading: as rock above wears away, curved sheets peel off a granite dome. Heat and cold: rock surfaces expand in the sun and shrink at night until they crack. Roots: a tree root grows into a crack and pries it apart. Abrasion: wind- or water-driven grains grind a rock face smooth. IceUnloadingHeat, coldRootsAbrasion freezing waterwedges cracks sheets peel offgranite domes expand by day,shrink by night growing rootspry rock apart moving grainsgrind it down

Mechanical weathering splits rock into smaller pieces made of the same minerals.

Every split exposes new surface, so chemistry has more rock to attack.

Part 1 · Key idea

Freezing water grows about 9% and pries cracks open

Frost wedging in three stages Stage 1: liquid water fills a narrow crack in a rock. Stage 2: the water freezes; the ice takes up about 9 percent more space and pushes the crack walls apart. Stage 3: after many freeze and thaw cycles, a piece breaks off. Below, two bars to scale: 100 cubic centimetres of liquid water becomes about 109 cubic centimetres of ice. 1 · Water fills2 · It freezes3 · It splits a crackice pushes the wallsafter many cycles water 100 cm³ ice ≈ 109 cm³

Ice takes up about 9% more space than the water it froze from.

Water seeps into a crack, freezes, and pushes the walls apart. Each freeze and thaw opens it a little more. This is frost wedging.

Ice is about 9% less dense than liquid water: USGS Water Science School, Water Density. Densities at 0 °C, water 0.9998 g/cm³ and ice 0.917 g/cm³ (CRC Handbook of Chemistry and Physics): 0.9998 ÷ 0.917 = 1.09.

Part 1 · Worked example · Force Sculptors' calculation

How much extra space does freezing water need?

Water and ice volumes, to scale Top: a bar for 20.0 cubic centimetres of water and a longer bar for 21.8 cubic centimetres of ice, drawn to scale, with the extra 1.8 cubic centimetres marked. Bottom: the build cup. Water filled to the 5.0 centimetre mark freezes into ice about 5.45 centimetres tall, 0.45 centimetres above the mark, if it can only grow upward. IN THE CRACK water 20.0 cm³ ice 21.8 cm³ +1.8 cm³ IN YOUR BUILD CUP 5.0 cm water ≈ 5.45 cm ice 5.0 cm mark ice ≈ 0.45 cm above (build test: ≥ 0.3 cm)
Water in the crack
20.0 cm³
On freezing
volume grows about 9% (× 1.09)
  1. Ice volume20.0 cm³ × 1.09 = 21.8 cm³
  2. Extra space21.8 cm³ − 20.0 cm³ = 1.8 cm³
  3. Check the percent1.8 ÷ 20.0 = 0.09 = 9%
  4. Your cup5.0 cm × 1.09 ≈ 5.45 cm, so ice rises about 0.45 cm.
Answer

The ice needs 21.8 cm³: 1.8 cm³ more than the crack holds. Check: a little more, never double.

Part 1 · Worked example · Practice Gym and Force Sculptors

Break a cube: how much new surface?

One cube split two and three ways per edge Three cubes, each 6 centimetres on an edge. The first is whole: 216 square centimetres of surface. The second is split 2 ways per edge into 8 cubes: 432 square centimetres. The third is split 3 ways per edge into 27 cubes: 648 square centimetres. Bars below show the surface doubling and tripling while the volume stays 216 cubic centimetres. 1 cube8 cubes27 cubes edge 6 cmedge 3 cmedge 2 cm 216 cm²432 cm²648 cm² surface area · volume stays 216 cm³
Cube edge
6 cm
Split
2 parts per edge (8 cubes), then 3 (27 cubes)
  1. Whole cube6 faces × 6 cm × 6 cm = 216 cm²
  2. 2 per edge8 cubes × 6 × 3 cm × 3 cm = 432 cm²
  3. 3 per edge27 cubes × 6 × 2 cm × 2 cm = 648 cm²
  4. The patternn parts per edge multiplies the surface by n: 216 × 3 = 648.
Answer

432 cm², then 648 cm². Check: the volume never changed (216 cm³); only the surface grew.

Practice Gym skill “Surface area gained by fracturing” and the Force Sculptors calculation in Watershed Forensics. Geometry only: real fragments are not perfect cubes.

Part 1 · Key idea · Lesson: Wind

Wind sorts grains by size

How wind moves dust, sand and gravel Wind blows from left to right over a mix of grains. Gravel stays behind at the upwind end as a coarse layer after the fines are blown away. Sand grains hop along close to the ground, usually no more than about 1 centimetre high, and pile into a dune downwind. Dust smaller than about 0.2 millimetres is lifted high and carried far, where it can settle as loess. wind dust: suspension grains under about 0.2 mm, carried high and far gravel stays behind (deflation took the fines) sand hops: saltation usually ≤ 1 cm high dune gentle upwind, steep downwind
Hop heights are drawn far larger than life so you can see them.

Air is thin, so wind can lift only small grains.

Dust floats in suspension. Sand hops in saltation. Deflation blows the fines away and leaves gravel behind. Dust that settles far downwind builds loess.

Part 1 · Key idea · Lesson: Glaciers

Glaciers scrape rock and drop unsorted till

What a glacier carves and drops A side view. A glacier flows down a valley. Rocks frozen into its base scrape grooves into the bedrock and pluck out blocks. At the front of the ice is a moraine, a ridge of till with boulders, gravel, sand and mud all mixed together. A meltwater stream flows away from the ice and drops outwash in sorted order: gravel nearest the ice, then sand, then fine silt farthest away. An inset shows the U-shaped cross-section of a glacier-carved valley. glacier flows downhill rocks in the ice scrape grooves and pluck blocks till: all sizes mixed moraine outwash, sorted: gravel → sand → silt U-shaped valley

A glacier is a thick body of ice that flows slowly downhill under its own weight.

Rocks in its base scrape grooves. The ice drops till, all sizes mixed, in ridges called moraines. Meltwater sorts grains into outwash.

Part 1 · Common mistakes

Three breaking-rock mistakes that cost points

Weathering stays in place; erosion moves the pieces in place moves
✗ Mistake

“Weathering and erosion are the same thing.”

✓ Fix

Weathering breaks rock where it sits. Erosion carries the pieces away.

Eight smaller cubes have twice the surface, not eight times × 2 ✓ × 8 ✗
✗ Mistake

“Split a cube into 8, and it has 8 times the surface.”

✓ Fix

Each piece is smaller too. Two parts per edge only doubles the surface.

Till is mixed; outwash is sorted into layers till: mixed outwash: sorted
✗ Mistake

“Everything a glacier drops is sorted by size.”

✓ Fix

Ice drops till unsorted. Only meltwater sorts grains, into outwash.

Part 1 · Check yourself

Your turn: surface area and frost

A 10 cm rock cube (600 cm² of surface) splits into 64 equal cubes, 4 per edge. What is the total surface now?

Show the answer

B, 2,400 cm². Four parts per edge multiplies the surface by 4: 600 × 4. Check: 64 cubes × 6 faces × 2.5 cm × 2.5 cm = 2,400. A forgets the new faces; C multiplies by 16; D multiplies by all 64 cubes.

A crack holds 50.0 cm³ of water. When it freezes, about how much extra space must the ice force open?

Show the answer

B, 4.5 cm³. 50.0 × 1.09 = 54.5 cm³ of ice, and 54.5 − 50.0 = 4.5 cm³ extra. A is the 9% written as a decimal, not a volume; C divides, as if ice shrank; D is the whole ice volume, not the extra.

Part 2 · Rock Rot lead this part

Rotting rock: chemistry, soil, and caves

  • Name three ways water changes minerals.
  • Explain why crushed rock reacts faster.
  • Calculate a reaction rate and a gas mass.
  • Read a soil profile from O to R.
  • Explain how groundwater dissolves a cave.
Soil layers above a limestone cave A soil profile, from a thin plant layer through dark topsoil, a pale leached layer and clay-rich subsoil, down to layered limestone. Inside the limestone is a cave with stalactites hanging from the ceiling and a stalagmite on the floor.

Part 2 · Key idea · Lesson: Chemical weathering

Chemical weathering turns minerals into new substances

Three kinds of chemical weathering Three panels. Hydrolysis: a feldspar crystal reacts with water and becomes flaky clay plus dissolved ions. Oxidation: an iron-bearing grain reacts with oxygen and grows a rust-coloured iron oxide coat. Carbonation: rain that has taken up carbon dioxide forms weak carbonic acid, which dissolves limestone; calcium and bicarbonate ions leave in solution and the surface is pitted. + O₂ + CO₂ Ca²⁺HCO₃⁻ HydrolysisOxidationCarbonation feldspar + water→ clay + ionsiron + oxygen→ rusty iron oxidelimestone + carbonicacid → ions in water feldsparclayiron grainrust coat

Chemical weathering changes what rock is made of.

In hydrolysis, water turns feldspar into clay. In oxidation, oxygen rusts iron. In carbonation, rain made slightly acidic by carbon dioxide dissolves limestone.

Part 2 · Key idea

Warm, wet places rot rock fastest

Chemical weathering speeds up with warmth and water A two-by-two picture, a classroom summary rather than measured data. Across: cold to warm. Up: dry to wet. A cold, dry rock is barely changed. Warm and dry, or cold and wet, the rock has a thin weathered rind. Warm and wet, the rock has rotted deep into clay-rich soil, with resistant quartz grains left in the clay. warmer → wetter → barely changed thin rind thin rind rotted deep into clay
Circles: quartz grains that outlast the rest. A classroom summary, not data.

Reactions run faster when it is warm and when water keeps flowing through rock.

Quartz resists these reactions. It outlasts the minerals around it, which is why so much beach sand is quartz.

Part 2 · Worked example · Rock Rot's calculation

How much faster does crushed chalk fizz?

Mass lost by crushed and whole chalk in 10 minutes Two cups of vinegar, each with 5.0 grams of calcite chalk: one crushed, one whole. A bar chart shows the mass lost as carbon dioxide gas in 10 minutes: 0.9 grams from the crushed chalk and 0.3 grams from the whole piece. A dashed line at 2.2 grams marks the most carbon dioxide 5.0 grams of calcite could ever give off. crushed whole piece CaCO₃ + acid → ions + H₂O + CO₂ gas the gas escapes, so the cup gets lighter crushed whole 0.9 g 0.3 g 2.2 g: the most 5.0 g can give 01.02.0 g mass lost in 10 minutes
Each cup
5.0 g of calcite chalk (CaCO₃) in vinegar
Lost in 10 min
crushed 0.9 g · whole 0.3 g
Gas ratio
100.09 g of calcite gives 44.01 g of CO₂
  1. Crushed rate0.9 g ÷ 10 min = 0.09 g/min
  2. Whole rate0.3 g ÷ 10 min = 0.03 g/min
  3. Compare0.09 ÷ 0.03 = 3 times faster
  4. Most gas5.0 g × 44.01 ÷ 100.09 = 2.2 g of CO₂
Answer

Crushed chalk reacts 3 times faster. At most 2.2 g of CO₂ can form. Check: 0.9 g and 0.3 g are both below 2.2 g.

Calculation: Rock Rot in Watershed Forensics. Formula masses from standard atomic weights (CRC Handbook): CaCO₃ 100.09 g/mol, CO₂ 44.01 g/mol. Test your chalk first: some chalk is gypsum and does not fizz.

Part 2 · Key idea · Lesson: How soil forms

Soil forms in layers, from leaf litter to bedrock

A soil profile, O to R An example soil profile in typical colours, layers not equal in thickness. O: brown leaf litter and humus on top. A: dark topsoil rich in organic matter. E: a pale layer that water has leached. B: reddish-brown subsoil where clay and iron oxides washed down from above collect. C: broken, partly weathered parent material. R: solid bedrock. Arrows show water carrying clay and dissolved minerals from the E layer down into the B layer. OAEBCR leaf litter, humustopsoil: dark, organicleached: palesubsoil: clay and ironoxides collect herebroken parent materialbedrock (not soil) example profile; real soils may lack some layers

Soil builds up in layers called soil horizons.

Water soaking down carries clay and dissolved minerals out of the upper layers and into lower ones. That is leaching. A real soil may be missing a layer.

Part 2 · Key idea

Five factors decide what soil you get

Five soil-forming factors around one soil A soil column in the centre with five factors around it: climate (sun and rain), organisms (roots and worms), relief (the slope of the land), parent material (the rock or sediment the soil formed from), and time (a clock). climate organisms relief slope of the land parent material rock or sediment time

Climate, organisms, relief, parent material (what the soil formed from), and time shape every soil.

Residual soil forms on its own bedrock. Transported soil forms in sediment that rivers, wind, or ice carried in.

HoustonOur soils formed in river-laid sediment.

Part 2 · Key idea · Lesson: Groundwater weathering and deposition

Groundwater dissolves limestone into caves

A karst landscape in cross-section Rain soaks through soil into limestone. The slightly acidic water widens cracks and dissolves a cave. At the surface, a sinkhole has formed where the ground sank into a dissolved space. Inside the cave, stalactites hang from the ceiling and stalagmites grow up from the floor where dripping water left calcite behind. Below, the water table and a spring where groundwater flows out. sinkhole acidic water widens cracks stalactites hang down stalagmites grow up water table spring limestone

Slightly acidic groundwater dissolves limestone, opening caves and sinkholes. Such landscapes are karst.

Drips that lose carbon dioxide leave calcite behind: stalactites hang down, stalagmites grow up.

TexasThe Edwards Aquifer is karst, so city runoff can reach it fast.

Part 2 · Common mistakes

Three rotting-rock mistakes that cost points

Feldspar becomes a new substance, clay feldspar clay
✗ Mistake

“Chemical weathering is just rock getting wet.”

✓ Fix

The minerals change into new substances, like feldspar into clay.

Sediment carried in from far away sits on unrelated bedrock carried in soil other rock
✗ Mistake

“Soil always matches the rock right under it.”

✓ Fix

Transported soil formed in sediment carried from somewhere else.

Crushed chalk makes more bubbles than one piece crushed whole
✗ Mistake

“A whole piece reacts as fast as crushed pieces.”

✓ Fix

Crushing exposes more surface, so the reaction runs faster.

Part 2 · Check yourself

Your turn: gas mass and soil layers

10.0 g of calcite dissolves completely in vinegar. What is the most carbon dioxide it can give off?

Show the answer

B, 4.4 g. 10.0 × 44.01 ÷ 100.09 = 4.4 g. A is the ratio 0.44 with the 10.0 g left out; C assumes all the calcite turns into gas; D divides the wrong way (10.0 × 100.09 ÷ 44.01).

Water washes clay down out of a pale layer. Which horizon collects that clay?

Show the answer

C, the B horizon. Clay and iron oxides leached from E collect in B, the subsoil. E is the pale layer they leave; A is dark topsoil above it; R is bedrock, which is not soil.

Part 3 · Sediment Trail lead this part

Carrying it away: runoff, streams, and sediment

  • Trace where rain goes, and why paving changes it.
  • Outline a watershed and find its outlet.
  • Calculate runoff volume and discharge, Q = A × v.
  • Read the Hjulström curve.
  • Predict where a stream drops its sediment.
A stream carries sediment into a lake A map view of a winding stream that carries sediment grains downhill and drops them where it enters a lake, building a small delta.

Part 3 · Key idea · Lesson: Surface water movement

Paving turns soaking rain into runoff

Where rain goes as more ground is paved Four bars, each 100 percent of the rain. Natural ground cover: 10 percent runoff, 40 percent evapotranspiration, 50 percent soaks in. 10 to 20 percent paved: 20 runoff, 35 evapotranspiration, 45 soaks in. 35 to 50 percent paved: 30 runoff, 35 evapotranspiration, 35 soaks in. 75 to 100 percent paved: 55 runoff, 30 evapotranspiration, 15 soaks in. runoff evapotranspiration soaks in Natural cover10–20% paved35–50% paved75–100% paved 10%40%50% 20%35%45% 30%35%35% 55%30%15% share of the rain that falls on the land
“Paved” means impervious: roofs, roads and lots that water cannot soak through.

Rain has three ways out. It soaks in (infiltration), flows across the surface (runoff), or returns to the air from soil and plants (evapotranspiration).

Pave the ground, and runoff rises from about 10% to about 55%.

Part 3 · Key idea

A watershed is all the land that drains to one outlet

A watershed seen from above A map view. A dashed line, the divide, circles the watershed along the high ground. Small streams join into one main channel that leaves through a single outlet at the bottom, where a stream gauge measures the flow. A raindrop landing just inside the divide flows into this watershed; a raindrop just outside it flows away into the next one. divide the ridge line drains in drains away outlet + gauge every drop inside the line leaves here

A watershed is all the land that drains to one outlet.

Its edge is the divide, a line of high ground where rain splits one way or the other.

HoustonBuffalo Bayou at the Houston gauge drains 336 square miles.

Part 3 · Worked example · Practice Gym model

How much water runs off a 10-hectare campus?

80 millimetres of rain split into runoff and soaking in A column of 80 millimetres of rain is split: 52 millimetres run off and 28 millimetres soak in, because 35 percent infiltrates. Beside it, a 10-hectare square catchment drains to one corner. Below, the unit rule: 1 millimetre of water over 1 hectare is 0.001 metres times 10,000 square metres, which is 10 cubic metres. 80 mm rain 52 mm runs off (65%) 28 mm soaks in (35%) 10 ha catchment = 100,000 m² UNIT RULE 1 mm on 1 ha = 0.001 m × 10,000 m² = 10 m³
Rain
80 mm
Catchment
10 ha
Soaks in
f = 0.35; no evapotranspiration during the storm
  1. Fraction that runs off1 − 0.35 = 0.65
  2. Runoff depth80 mm × 0.65 = 52 mm
  3. Depth × area52 mm × 10 ha = 520 mm·ha
  4. Convert520 × 10 m³ = 5,200 m³
Answer

5,200 m³ of runoff. Check: 28 mm soaks in (2,800 m³), and 5,200 + 2,800 = 8,000 m³ = 80 × 10 × 10.

Practice Gym skill “Partition rainfall into runoff volume,” a classroom model: the storm's rain splits only into runoff and infiltration. Over a whole year, evapotranspiration is often the largest share (see the paving data).

Part 3 · Key idea

Paving makes the flood peak higher and sooner

Storm hydrographs for a forested and a paved watershed A schematic graph of stream flow against time after the same storm. Rain bars hang from the top early on. The paved watershed's flow rises fast to a high, narrow peak soon after the rain. The forested watershed's flow rises slowly to a lower, broader peak much later. The lag time, from the middle of the rain to the peak, is short for the paved watershed and long for the forest. time after the rain starts flow in the stream rain paved: higher, sooner forest: lower, later short lag long lag
Schematic shapes, like the Watershed Sandbox's Flood Hydrograph. Read real ones on a USGS gauge.

A hydrograph graphs a stream's flow over time.

Paved ground sends rain to the channel fast, so the peak comes sooner and higher. The delay from rain to peak is the lag time.

Part 3 · Key idea

Discharge: how much water passes each second

Discharge equals area times velocity Left: a channel cross-section with width w and depth d; its area A is width times depth, and the water moves through it at velocity v. Right: a channel seen from above that narrows to half its width. The same discharge passes both parts, so the arrows in the narrow part are twice as long: the water moves twice as fast. width w d A = w × d v looking upstream: water flows toward you wide: slower narrow: faster Q = A × v

Discharge is the volume of water passing a point each second: Q = A × v.

A is the channel's cross-section area, width × depth. v is the water's speed. The same Q through half the area moves twice as fast.

Streamflow is measured as area × velocity: USGS, How Streamflow is Measured. Practice Gym skill “Stream discharge Q = A·v” uses a rectangular channel, A = w × d.

Part 3 · Worked example · Sediment Trail's calculation

How fast is the bayou flowing?

Two channel cross-sections carrying the same discharge A gauge reads 530 cubic feet per second, which is 15.0 cubic metres per second. Two cross-sections drawn to the same scale: a channel 20 metres wide and 3 metres deep, area 60 square metres, where the water moves at 0.25 metres per second; and a narrower part 10 metres wide and 3 metres deep, area 30 square metres, where the same water moves at 0.50 metres per second. GAUGE READING 530 ft³/s × 0.0283 = 15.0 m³/s 20 m × 3 m = 60 m² v = 0.25 m/s 10 m × 3 m = 30 m² v = 0.50 m/s wide part narrow part: twice as fast cross-sections drawn to the same scale
Gauge
530 ft³/s; 1 ft³/s = 0.0283 m³/s
Channel
20 m wide, 3 m deep; then 10 m wide
  1. Convert530 × 0.0283 = 15.0 m³/s
  2. Wide area20 m × 3 m = 60 m²
  3. Wide speed15.0 ÷ 60 = 0.25 m/s
  4. Narrow area10 m × 3 m = 30 m²
  5. Narrow speed15.0 ÷ 30 = 0.50 m/s
Answer

0.25 m/s, then 0.50 m/s. Check: 60 × 0.25 = 30 × 0.50 = 15.0 m³/s.

Part 3 · Worked example · Practice Gym

How steep is the river, on average?

A river's long profile from source to mouth A side view of a river 60 kilometres long that drops from 420 metres at its source to 0 metres at its mouth. The real profile curves: steep near the source, gentle near the mouth. A dashed straight line between the ends shows the average gradient, 420 metres in 60 kilometres. The vertical scale is stretched about 78 times. source: 420 m mouth: 0 m 60 km of channel average: 420 m ÷ 60 km steep near the source gentle near the mouth
Vertical scale stretched about 78 times so the drop shows.
Drop
420 m
Channel length
60 km
  1. Gradient = drop ÷ length420 m ÷ 60 km = 7.0 m/km
  2. Same units7.0 m ÷ 1,000 m = 0.007
  3. As a percent0.007 × 100 = 0.7% grade
Answer

7.0 m/km, a 0.7% grade. Check: 7.0 m/km × 60 km = 420 m.

Practice Gym skill “Stream gradient = drop ÷ length”; example from the Module 2 Practice Blueprint. A steeper gradient gives water more energy per metre, so it flows faster and erodes more.

Part 3 · Key idea · Watershed Sandbox model

Fine sand moves first, not clay

Hjulström curve: water speed needed to move each grain size Log-log chart of water speed in centimetres per second against grain diameter in millimetres. The pick-up line is lowest, about 20 centimetres per second, for fine sand near 0.3 millimetres. It rises for coarser grains, to about 90 centimetres per second for 10 millimetre gravel, and it also rises for finer grains: about 62 for 0.02 millimetre silt and about 150 for packed clay at 0.001 millimetres, because clay sticks together. Above the line grains are picked up; between the lines they are carried; below the settling line they settle out. picked up (erosion) carried along settles out (deposition) fine sand ≈ 0.3 mm moves at ≈ 20 cm/s packed clay ≈ 150 cm/s settling line 10001001010.1 0.0010.010.11101001000 claysiltsandgravel grain diameter (mm, log scale) water speed (cm/s, log scale)

Fine sand is the easiest sediment to pick up, at about 20 cm/s.

Smaller grains need faster water again, because packed clay sticks together. Bigger grains need faster water too.

Part 3 · Key idea

Competence is the biggest grain; capacity is the total load

How a stream carries its load A channel cross-section. Dissolved ions travel invisibly in the water. Fine mud and silt hang in the water as suspended load. Sand and gravel roll and bounce along the bottom as bed load. Below: competence, the biggest grain the stream can move, is set by its speed; capacity, the total amount it can carry, is set by its discharge. +−+−+− dissolved suspended bed load COMPETENCE the biggest grain set by the water's speed CAPACITY the total amount set by the discharge

A stream's load travels three ways: dissolved, suspended in the water, and rolling along the bed.

Competence is the biggest grain the flow can move. Capacity is the total load it can carry.

Part 3 · Key idea · Lesson: Erosion and deposition

Water drops its sediment where it slows down

Erosion and deposition along a bend and at a lake A map view of a winding stream. At a bend, the fast water on the outside cuts back the bank, called a cut bank, while slow water on the inside drops sand in a point bar. Where the stream enters a lake it slows and builds a delta: gravel and coarse sand settle first near the mouth, and fine silt and clay settle farthest out. outside: fast water cuts the bank back inside: slow water drops sand (point bar) lake delta: coarse first, fine mud farthest

Fast water erodes. Slow water drops its load, biggest grains first. Dropping sediment is deposition.

On a bend, the fast outside is cut back and the slow inside gains sand. At a lake, the stream builds a delta.

Part 3 · Key idea · Lesson: Streams, lakes, and wetlands

Wetlands slow floods and trap sediment

A wetland slows a stream, and flood peaks drop Left: muddy water enters a wetland, spreads out over a wide flat area among plants, slows, and drops its sediment, then leaves clearer and later. Right: a bar chart. Flood peaks in basins with 30 percent or more of their area in lakes and wetlands were 60 to 80 percent lower than in basins without wetlands. muddy in clearer out, and later spreads out, slows, drops sediment Flood peak 100% 20–40% no wetlands ≥ 30% lakes and wetlands 60–80% lower

A wetland is land soaked or covered by shallow water for much of the year.

It spreads floodwater over wide, flat ground. The water slows, drops its sediment, and leaves later and cleaner.

Part 3 · Common mistakes

Three runoff and stream mistakes that cost points

The Hjulström curve dips lowest at sand, not clay sand: lowest clay gravel
✗ Mistake

“Clay is the easiest sediment to erode.”

✓ Fix

Fine sand moves first, near 20 cm/s. Packed clay sticks together and needs about 150.

The narrow part of a channel flows faster faster
✗ Mistake

“A narrower channel slows the water down.”

✓ Fix

Same discharge through less area means faster water: Q = A × v.

Multiply millimetres times hectares by 10 to get cubic metres 52 × 10 = 520 m³ ✗ 52 × 10 × 10 = 5,200 m³ ✓
✗ Mistake

“52 mm on 10 ha is 520 m³.”

✓ Fix

Convert first: 1 mm on 1 ha is 10 m³, so it is 5,200 m³.

Part 3 · Check yourself

Your turn: discharge and the Hjulström curve

A channel 12 m wide and 1.5 m deep flows at 0.8 m/s. What is its discharge?

Show the answer

B, 14.4 m³/s. A = 12 × 1.5 = 18 m², and Q = 18 × 0.8 = 14.4 m³/s. A multiplies width by speed and skips depth; C is the area alone; D divides the area by the speed.

On the Hjulström curve, which grain starts moving in the slowest current?

Show the answer

C, fine sand. On the Sandbox curve it moves at about 20 cm/s. Clay needs about 150 cm/s because it sticks together, silt about 62, and 10 mm gravel about 90 because it is heavy.

Part 4 · Soak and Slide lead this part

Soaking in and sliding

  • Explain why sand soaks up water faster than clay.
  • Calculate porosity and a Factor of Safety.
  • Explain why slopes slide after heavy rain.
  • Find which way groundwater flows, and how steeply.
  • Explain why pumping made Houston sink.
A soaked slope slumping above the water table A cross-section of a hillside. A soaked block of soil has slumped along a curved surface, rain is falling, a dashed water table runs below the slope, and a well reaches down to it.

Part 4 · Key idea · Lesson: Soil texture and water

Soil texture sets how fast water soaks in

Grain sizes, and how fast rain soaks in Top: the USDA grain-size classes. Sand is 0.05 to 2 millimetres, silt 0.002 to 0.05 millimetres, and clay smaller than 0.002 millimetres; circles show the relative sizes, not to scale. Bottom: a bar chart of how fast rain soaks in. Up to about 25 millimetres per hour in mature forest on sandy soil, a few millimetres per hour in clayey and silty soils, and zero on pavement. GRAIN SIZE (USDA) sandsiltclay 0.05–2 mm0.002–0.05 mmunder 0.002 mm not to scale HOW FAST RAIN SOAKS IN (mm per hour) sandy forest soil ≈ 25 clayey, silty soil a few pavement 0

Soil texture is the mix of sand, silt, and clay in a soil.

Sand grains leave big gaps, so water soaks in fast. Clay's tiny grains leave tiny gaps, so water soaks in slowly.

Part 4 · Key idea

Holding water is not the same as letting it flow

Porosity and the water that drains out Paired bars, in percent of the volume. Clay: porosity 50 percent, but only 2 percent drains out by gravity. Sand: porosity 25 percent, and 22 percent drains out. Gravel: porosity 20 percent, and 19 percent drains out. porosity: open space drains out by gravity 0%15%30%45%60% 50225222019 claysandgravel holds a lot, gives little

Porosity: how much of a material is open space. Permeability: how easily water flows through it.

Clay is half open space, yet gives up almost no water. An aquifer is a layer that holds water and lets it flow.

Part 4 · Worked example · Soak and Slide's calculation

How much of your sand is empty space?

Filling the spaces in 100 millilitres of dry sand A measuring cup holds 100 millilitres of dry sand. Water poured in slowly fills the spaces between grains; 34 millilitres just reach the top of the sand. A scale on the right marks 34 percent between two reference packings of equal spheres: 26 percent for the tightest packing and 48 percent for the loosest. 100 mL of dry sand + 34 mL of water fills it 0%30%60% 26% tightest 48% loosest 34%: your sand equal spheres
Dry sand
100 mL
Water to fill it
34 mL, just to the top of the sand
  1. Open spaceThe water filled the gaps: 34 mL of pore space.
  2. Porosity34 mL ÷ 100 mL = 0.34
  3. As a percent0.34 × 100 = 34%
Answer

34% porosity. Check: between tight (26%) and loose (48%) packings, and inside the build's 25–45 mL target.

Part 4 · Key idea · Lesson: Mass movements

Mass movements range from slow creep to sudden rockfall

Four kinds of mass movement, slowest to fastest Four panels on a speed arrow from imperceptibly slow to free fall. Creep: soil inches downhill, tilting fence posts and curving tree trunks. Slump: a block of soil slides along a curved surface and rotates. Debris flow: soaked soil, rock and water race down a gully as a slurry and spread in a fan. Rockfall: blocks break off a cliff and fall, bounce and roll. CreepSlumpDebris flowRockfall tilts posts, treesblock slides ona curved surfacesoaked slurryraces down imperceptibly slow: years free fall: seconds

Mass movement is rock and soil moving downhill under gravity, with no river or glacier carrying it.

Creep takes years. A slump slides a block on a curved surface. A debris flow races down as a soaked slurry.

Part 4 · Key idea

A slope fails when the pull beats the grip

Forces on a block of soil on a slope A block of soil sits on a slope at angle theta. Its weight points straight down. Part of the weight pulls it down the slope; the other part presses it into the slope, and that pressing creates friction, the grip that holds it. Factor of Safety equals grip divided by pull. For dry, loose sand this is tan phi divided by tan theta, where phi is the friction angle. θ presses in: sets the grip weight pull down the slope grip friction holds it FACTOR OF SAFETY FS = grip ÷ pull dry, loose sand: FS = tan φ ÷ tan θ φ friction angle · θ slope angle

Factor of Safety (FS) = forces holding a slope ÷ forces pulling it down.

Above 1, it holds. Below 1, it fails. Dry, loose sand fails once the slope passes its friction angle, about 30–34°. That steepest stable angle is the angle of repose.

Part 4 · Worked example · Practice Gym and Soak and Slide

Which of these slopes will stand?

Factor of Safety against slope angle for a friction angle of 32 degrees A curve of Factor of Safety, tan 32 degrees divided by tan theta, against slope angle from 12 to 45 degrees. It falls as the slope steepens and crosses 1 at exactly 32 degrees. Marked points: 20 degrees, FS 1.72; 25 degrees, 1.34; 30 degrees, 1.08, all stable; 35 degrees, 0.89, fails. Below FS 1 the region is shaded as failure. 1.721.341.080.89 crosses 1 at 32° fails holds 10°20°30°40° 0123 slope angle θ Factor of Safety
Friction angle
φ = 32° (dry, loose sand)
Slope angles
20°, 25°, 30°, 35°
  1. Griptan 32° = 0.625
  2. Pull at 25°tan 25° = 0.466
  3. FS at 25°0.625 ÷ 0.466 = 1.34, above 1: holds
  4. FS at 35°0.625 ÷ 0.700 = 0.89, below 1: fails
  5. The others20°: 1.72 · 30°: 1.08, both hold
Answer

Only the 35° slope fails. Check: FS is above 1 exactly when the slope angle is less than 32°.

Practice Gym skill “Slope stability: Factor of Safety” (FS = tan φ ÷ tan θ, a dry, loose-slope classroom model) and the Soak and Slide calculation in Watershed Forensics.

Part 4 · Key idea

Water soaking into a slope loosens its grip

Grains in a dry slope and in a soaked slope Two close-ups of sand grains. Dry: the grains press firmly on each other, shown by large contact arrows, so friction between them is high. Soaked: water fills the pores and pushes outward on the grains, carrying part of the weight, so the grains press together less and the grip is smaller. A third sketch shows roots threading through soil, adding grip back. DrySoakedRoots grains press hardwater pushes them aparttie the soil together strong gripweak gripgrip added close-ups, not to scale

Rain fills the spaces between grains. That pore water carries part of the weight, so grains press together less and grip less.

Roots add grip back. Bare, soaked slopes are the ones that slide after storms.

Part 4 · Key idea · Lesson: Groundwater

Groundwater fills the pores below the water table

The water table under hills and a stream A cross-section through a hill and a stream valley. Above the water table, the unsaturated zone has pores holding both air and water. Below it, in the saturated zone, every pore is full of water. The water table is higher under the hill and dips down to meet the stream, which groundwater slowly feeds. A well reaches below the water table. A circular close-up shows water filling the spaces between sand grains: no open river. unsaturated zone pores hold air and water saturated zone every pore full of water water table well stream water fills pores

Below the water table, every pore is full of water. That is the saturated zone.

The water table rises under hills and meets streams in valleys. Groundwater seeps slowly through pores. It is almost never an underground river.

Part 4 · Worked example · Practice Gym

Which way, and how steeply, does groundwater flow?

Water levels in two wells 400 metres apart Two wells 400 metres apart. Water stands at 210 metres elevation in Well A and 198 metres in Well B, so the water table slopes down from A to B by 12 metres. Groundwater flows from A toward B. The vertical scale is stretched. Well A Well B 210 m 198 m groundwater flows A → B 400 m apart
Vertical scale stretched so the 12 m drop shows.

Head is the height water stands at in a well. The hydraulic gradient is the drop in head per distance.

Well A
210 m
Well B
198 m, 400 m away
  1. Head drop210 m − 198 m = 12 m
  2. Gradient12 m ÷ 400 m = 0.030
  3. As a percent0.030 × 100 = 3.0%
Answer

3.0%, flowing from A toward B, high head to low. Check: 0.030 × 400 m = 12 m.

Part 4 · Key idea · Houston

Pumping groundwater made Houston sink

How pumping groundwater lowers the land Before and after, not to scale. Before pumping: sand aquifers between clay layers, with water standing high in a well. After pumping: water levels fell by as much as 350 feet, the clay layers drained and squeezed thinner, and the land surface sank. Near Pasadena it sank more than 10 feet between 1906 and 1995. A dashed line marks where the ground used to be. Before pumping After decades of pumping land sank > 10 ft near Pasadena, 1906–1995 water level fell up to 350 ft clay sand aquifer clay squeezed not to scale old ground level

Pumping lowered Houston-area water levels by as much as 350 feet. Drained clay layers squeezed together, and the land sank.

That sinking is subsidence: more than 10 feet near Pasadena from 1906 to 1995. Lower land floods more often.

Part 4 · Common mistakes

Three soil-water and slope mistakes that cost points

Groundwater fills pores between grains water in pores
✗ Mistake

“Groundwater flows in underground rivers.”

✓ Fix

It seeps slowly through pores and cracks. Cave streams need dissolved limestone.

FS is grip divided by pull pull ÷ grip ✗ grip ÷ pull ✓
✗ Mistake

“FS = pull ÷ grip, so a big FS means danger.”

✓ Fix

FS = grip ÷ pull. Above 1 the slope holds; below 1 it fails.

Clay holds water but sand gives it up clay sand
✗ Mistake

“Clay holds the most water, so it is the best aquifer.”

✓ Fix

Clay barely lets water flow. Sand and gravel give it up.

Part 4 · Check yourself

Your turn: slope stability and groundwater flow

A dry, loose slope has friction angle 34° and slope angle 28°. FS = tan 34° ÷ tan 28°. What is FS, and does the slope hold?

Show the answer

A. tan 34° = 0.675 and tan 28° = 0.532, so FS = 1.27, above 1: it holds. B divides the wrong way (pull ÷ grip); C divides the angles, 34 ÷ 28, without tan; D has the right number but reads it backwards.

The water table stands at 250 m in Well A and 241 m in Well B, 600 m away. What is the hydraulic gradient?

Show the answer

B, 1.5%. 250 − 241 = 9 m, and 9 ÷ 600 = 0.015 = 1.5%, flowing from A to B. A labels the decimal as a percent; C divides 9 by 250, a height, not a distance; D writes the 9 m drop as a percent.

Part 5 · Every team · Watershed Forensics

Use it in your project

“Where does the rain that lands on this campus end up, and who downstream pays for what we do to it?”

  • Trace your watershed and predict a hotspot.
  • Turn a human change into measurable criteria.
  • Compare two real fixes fairly.
  • Find the slides your team's build needs.
Open Watershed Forensics Project Library
The four Watershed Forensics deliverables Four cards: a trace map with a divide, a stream and a hotspot star; a typed findings brief with numbers; a trade-off table; and a field journal page. trace map findings brief m³ trade-off table field journal

Use it · Watershed Forensics · Watershed trace map

Trace your watershed from ridge to outflow

What a strong trace map shows An example trace map. A dashed divide circles the watershed. Arrows show flow from the ridge down to the outlet. Land cover is shaded: forest in the upper part, paved neighbourhoods and a campus lower down. One hotspot star sits where the channel widens and slows below a paved area, labelled with its reason: fast runoff from pavement meets slower water, so sediment drops. A legend and a north arrow complete the map. HOTSPOT deposition here: fast runoff from the pavement meets a wider, slower channel (predicted first) divide flow forest paved outlet gauge N
Turn in
Watershed trace map · Studio Turn-In, image pages
Do
Mark the divide, flow arrows, and land cover. Predict one erosion or deposition hotspot from slope, surface, and flow, then check it.
Data
USGS National Map · USGS gauge 08074000 · Web Soil Survey
Slides
Watershed · Paving · Where sediment drops

Use it · Watershed Forensics · Findings brief on one human-caused change

Turn “reduce flooding” into numbers someone can check

From a vague goal to measurable criteria and constraints Top card, vague: reduce flooding, make the bayou cleaner. Arrow down. Bottom left card, criteria, each with a number: hold the runoff from an 80 millimetre storm on our 10 hectare campus, about 5,200 cubic metres if 35 percent soaks in; keep the bayou peak below a stated flow by a stated year. Bottom right card, constraints: a cost ceiling in dollars, the land area it may use, and whose needs it must respect. ✗ VAGUE “Reduce flooding.” “Make the bayou cleaner.” ✓ CRITERIA: WHAT IT MUST DO Hold the runoff from an 80 mm storm on our 10 ha campus: ≈ 5,200 m³ (if 35% soaks in) Keep the bayou peak below ___ ft³/s at our gauge by ___ (year) CONSTRAINTS: LIMITS Cost under $___ Uses at most ___ ha Keeps the field usable Who must agree: neighbours, district
The 5,200 m³ comes from the runoff worked example. Fill every blank from a cited source.
Turn in
Findings brief · Assessment Center, typed
Do
Name one human-caused change and the evidence it happened. List criteria (what a fix must achieve) and constraints (limits it must respect), each with a number.
Start with
“The change is ___. Our evidence is ___.”
Slides
Runoff volume · Paving data · Subsidence

Use it · Watershed Forensics · Mitigation trade-off evaluation

Compare two real fixes against the same ranked criteria

Example layout. Every cell in your table cites a source.
Criterion (ranked)Option A: restored wetland or floodplainOption B: lined, straightened channel
1 · Big-storm waterStores and slows it (USGS: peaks 60–80% lower).Moves it out fast; the peak hits downstream sooner.
2 · SedimentTraps sediment as the water slows.Carries sediment straight through.
3 · Cost$ from your source, with the year$ from your source, with the year
4 · Upkeepwhat must be done, how oftenwhat must be done, how often
5 · Who is worse offWhoever loses the use of that land.Neighbours downstream, who get the water sooner.
Rank first

Order your criteria before you score either option.

Who pays

Name who is worse off under each option, not only the cost.

Own it

Recommend one, and state the trade-off you accept.

Use it · Watershed Forensics · Field journal

Log the criteria before the options

The seven parts of one journal entry A journal page with seven numbered parts: 1 header with date, day and role; 2 today's question; 3 raw record, numbers with units; 4 method log; 5 sense-making, the claim right now; 6 dead ends, required; 7 handoff to tomorrow. 1234567 HeaderToday's questionRaw recordMethod logSense-makingDead endsHandoff date, project day, team, your roleone sentence this day must settlenumbers with units, as takenwhat you did, and every change of planyour claim right now, and its evidencewhat failed, and why (required)next step, and tomorrow's question
Turn in
Field journal · Note Studio, one note titled “Watershed Forensics”
Do
One dated entry per project day, in seven parts, plus a closing retrospective.
Watch
Pick a favourite fix first and invent criteria to fit it, and your journal shows it.
Sandbox
“When we changed ___, the ___ went up by ___.”

Use it · Watershed Forensics · Your team's build

The slides your build and teach-back need

Force Sculptors · sculptor workbench

Frost cup, wind tunnel, ice-block glacier.

Rock Rot · rock-rot bench

Fizz test, soil column, sugar-cube cave.

Sediment Trail · stream table

Delta, cut bank, sponge wetland.

Soak and Slide · soak-and-slide tank

Drain cups, slope, porosity, straw well.

Watershed SandboxSandbox readouts are model evidence: “When we changed ___, the ___ changed.” Open the Sandbox

Use it · The rest of the year

Module 2 comes back in every project

Part 6 · Reference · Formula sheet

Every Module 2 formula on one page

Discharge

Q = A × v

A = width × depth (m²); v in m/s; Q in m³/s. 1 ft³/s = 0.0283 m³/s. Example

Runoff volume (Practice Gym)

V = R × (1 − f) × A × 10

R rain in mm, f fraction that soaks in, A in ha, V in m³. 1 mm on 1 ha = 10 m³. Example

Stream gradient

drop ÷ channel length

m/km. ÷ 1,000 for a slope; × 100 for a % grade. Example

Hydraulic gradient

head drop ÷ distance

Both in m; × 100 for %. Water flows from high head to low. Example

Factor of Safety (Practice Gym)

FS = tan φ ÷ tan θ

Dry, loose slope. FS > 1 holds; FS < 1 fails. Sandbox adds cohesion, roots and pore water. Example

Porosity

pore volume ÷ total volume × 100

1 mL = 1 cm³. Equal spheres: 26% tightest to 48% loosest. Example

Surface area

cube: 6s² · n parts per edge: × n

Volume does not change; only exposed surface grows. Example

Frost expansion

ice volume ≈ water volume × 1.09

Density: water 1.00 g/cm³, ice 0.917 g/cm³. Example

Rate and gas

rate = mass lost ÷ time

CO₂ from calcite = mass × 44.01 ÷ 100.09 (≈ × 0.44). Example

Water budget

rain = runoff + infiltration + evapotranspiration

± storage. Paving: runoff ≈ 10% → ≈ 55%. Data

Hjulström rule card (Sandbox)

fine sand moves first

Pick-up speed: sand 0.3 mm ≈ 20 cm/s · silt 0.02 mm ≈ 62 · packed clay ≈ 150 · gravel 10 mm ≈ 90. Chart

Reference values

Grain size (USDA): clay < 0.002 mm · silt 0.002–0.05 · sand 0.05–2. Porosity: clay 50% · sand 25% · gravel 20%. Dry sand stands at ≈ 30–34°.

Part 6 · Reference · Vocabulary, Parts 1–2

Module 2 words, in plain language (1 of 2)

Weathering
Breaking or changing rock where it sits.
Erosion
Carrying broken rock away by water, wind, ice, or gravity.
Mechanical weathering
Breaking rock into pieces of the same minerals.
Frost wedging
Freezing water expanding in a crack and prying it open.
Suspension
Fine grains carried along inside moving air or water.
Saltation
Grains hopping along the ground or bed.
Deflation
Wind removing loose fine grains, leaving coarser ones.
Loess
Wind-blown silt that settled in a thick layer.
Glacier
A thick body of ice flowing downhill under its own weight.
Till
Unsorted sediment dropped directly by ice.
Moraine
A ridge or mound of till.
Outwash
Sediment sorted and laid down by glacial meltwater.
Chemical weathering
Changing rock's minerals into new substances.
Hydrolysis
Water reacting with a mineral, such as feldspar into clay.
Oxidation
Oxygen reacting with iron in minerals: rusting.
Carbonation
Weak carbonic acid in rain dissolving limestone.
Soil horizon
One layer of a soil profile (O, A, E, B, C; R is bedrock).
Leaching
Water carrying clay and dissolved minerals down through soil.
Parent material
The rock or sediment a soil formed from.
Residual soil
Soil formed on its own bedrock.
Transported soil
Soil formed in sediment carried in from elsewhere.
Karst
Landscape of caves and sinkholes in dissolved rock.
Sinkhole
A hole where the ground sank into a dissolved space.
Stalactite
Mineral icicle hanging from a cave ceiling.
Stalagmite
Mineral mound growing up from a cave floor.

More wordsLook up any term in the Ascent Lexicon. Parts 3–5 are on the next slide.

Part 6 · Reference · Vocabulary, Parts 3–5

Module 2 words, in plain language (2 of 2)

Infiltration
Water soaking into the ground.
Runoff
Water flowing over the surface instead of soaking in.
Evapotranspiration
Water returning to the air from soil, water, and plants.
Watershed
All the land that drains to one outlet.
Divide
High ground separating one watershed from the next.
Hydrograph
A graph of a stream's flow over time.
Lag time
Delay between the rain and the flood peak.
Discharge
Volume of water passing a point each second (Q).
Load
Everything a stream carries: dissolved, suspended, bed load.
Competence
The biggest grain a flow can move.
Capacity
The total load a flow can carry.
Deposition
Dropping sediment where the carrier slows.
Delta
Sediment built up where a stream enters still water.
Wetland
Land soaked or under shallow water much of the year.
Soil texture
The mix of sand, silt, and clay in a soil.
Porosity
The share of a material that is open space.
Permeability
How easily water flows through a material.
Aquifer
A layer that holds water and lets it flow to wells.
Mass movement
Rock and soil moving downhill under gravity.
Creep
Very slow downhill movement of soil over years.
Slump
A block sliding and rotating on a curved surface.
Debris flow
A fast, soaked slurry of soil, rock, and water.
Factor of Safety
Forces holding a slope ÷ forces pulling it down.
Friction angle
Steepest angle grains can rest at before sliding.
Angle of repose
Steepest stable angle of a dry, loose pile.
Pore water
Water filling the spaces between grains.
Water table
Top of the zone where pores are full of water.
Saturated zone
Ground where every pore is full of water.
Head
The height water stands at in a well.
Hydraulic gradient
Drop in head per distance.
Subsidence
Land sinking, here from pumping groundwater.
Criteria
What a fix must achieve, stated with numbers.
Constraints
Limits a fix must respect: cost, space, people.

More wordsLook up any term in the Ascent Lexicon.

Part 6 · Reference · Trustworthy data

Real data for your project, from the people who measure it

Cite itWrite the source, the link, and the date you read it. Live data changes.

Part 6 · Ready for your project?

Three last checks

A granite boulder has rotted soft, but it sits exactly where it formed. What happened?

Show the answer

B. The rock changed but did not move: weathering in place. A, C and D all need the pieces to be carried somewhere.

50 mm of rain falls on a 4 ha lot and 20% soaks in. How much runs off?

Show the answer

C. 50 × 0.80 = 40 mm runs off; 40 × 4 × 10 = 1,600 m³. A skips the × 10; B is the part that soaks in; D is all the rain.

Where on a winding stream would you predict a deposition hotspot?

Show the answer

B. Water slows on the inside of a bend and drops sand. The outside, steep stretches and narrow spots all run fast, so they erode.

You made it through Module 2

Now trace your own watershed

Practice the math, test ideas in the Sandbox, then build.

Watershed Forensics project Practice Gym · Module 2 Watershed Sandbox Module 2 page
A hand lens over a stream map A hand lens magnifies part of a stream map, showing the channel and sediment grains on its bed.