- Stream/River- water flow down channels
- Runoff- water flow over the land surface.
- Stream runoff is crucial for huamans:
- Drinking water.
- Waste disposal.
Stream runoff also..
Stream runoff also causes many problems
- Flodding destorys live and property
The Hydrologic Cycle: Stream Flow
Stream flow- imp. component of hydrologic cycle.
Streamflow begins as water is added to the surface
Streamflow begins as moving sheetwash.
-Thin surface water layer.
-Moves down steepest slope.
- Sheetwash erosion creates tiny channels (rills).
- Rills coalesce & deepen into channels.
Scouring can mark entry into the channel.
Rapid erosion lengthens channel up slope.
---Process is called headward erosion.
Over time, channels merge.
- Smaller tributaries join a larger trunk system.
A drainage network- array of linked channels.
- They change over time.
Drainage networks form geometric patterns.
Patterns reflect geology & landscape form.
Several common drainage patterns:
1. Dendritic- Branching, "treelike"- due to uniform material.
Common Drainage Pattern: radial
Common drainage patterns:
2. Radial - From a point uplift (e.g. volcano)
Common Drainage Pattern: rectangular
3. Rectangular- Controlled by jointed rocks
Common Drainage Pattern: trellis
4. Trellis (garden)- due to alternating resistant/weak rocks.
-- Common in fold-thrust belts.
A Drainage Basin
Land area that drains into a specific trunk stream.
--> also called a catchment or watershed.
Divides are boundaries that separate drainage basins.
Watersheds exist across scales.
- Tiny tributaries.
- Continental rivers.
- Feed large rivers.
- Section continents.
Continental divides separate flow to different oceans.
Permanent vs. Ephemeral
Permanent streams" •
Water flows all year." •
At or below the water table." •
Humid or temperate."
• Sufficient rainfall."
• Lower evaporation." •
Discharge varies seasonally."
Do not flow all year."
Above the water table."
• Low rainfall."
• High evaporation."
Flow mostly during rare flash floods."
Amount of water flowing in a channel.
-Water volume passing a point per unit time.
---Cubic meters per second (m^3/s)
Given by cross-sectional area (Ac) x flow velocity.
Varies seasonally due to precipitation & runoff.
Velocity is not uniform in the channel.
-Friction slows water along edges.
---Greater in wider, shallower streams.
----Lesser in narrower, deeper streams.
--- Mangnitude determined by wetted perimeter
----------Greater wetted perimeter, slower the velocity.
In straight channels, highest velocity in center.
Velocity is not uniform within a channel.
- Max. velocity near outside in bending channels.
--- Outside is preferentially scoured &deepend (cut bank).
--- Inside is locus of deposition (point bar) due to reduced velocity.
--- Deepest part is called the thalweg.
Velocity is not uniform in all areas of a channel.
- Stream flow is turbulent.
---Chaotic & erratic.
Turbulence caused by...
- Shear in water.
Eddies scour channel bed.
River flow does work.
- Energy imparted is dervied from gravity.
- Do work by converting potential to kinetic energy.
Erosion is maximized during floods.
- Large water volumes, high velocities, abundant sediment.
Stream erosion- scour, break, abrade, dissolve, material.
1. Scouring- running water picks up sediment & moves it.
2. Breaking & lifting- The force of moving water can...
---Break chunks off the channel bottom/walls.
-Lift rocks off the channel bottom.
3. Abrasion- Sediment grains in flow "sandblast" rocks.
- Exposed bedrock in channels gets polished smooth.
- Gravel swirled by turbulent eddies drills holes.
---Bowl-shaped depressions are called called potholes.
---Potholes are unusual, intricately sculpted.
4. Dissolution- Mineral matter dissolves in water.
Sediment load- material moved by rivers.
1. Dissoved load- Ions from mineral weathering.
2. Suspended load- Fine praticles (silt & clay) in teh flow.
3. Bed load- Large particles roll, slide, bounce along bottom.
Competence- maxium size transported.
Capacity- maximum load transported.
- Change with discharge:
---High discharge- Large cobbles & boulders may move.
--- Low discharge- Large clasts are stranded.
When flow velocity decreases..."
Competence is reduced & sediment drops out.
" Grain sizes are sorted by water. "
• Sands are removed from gravels; muds from both. " • Gravels settle in channels.
" • Sands drop out in near channel environments."
• Silts & clays drape floodplains away from channels."
Sediment size tracks with river slope.
- Coarsest particles typify steep gradients in headwaters.
- Fine particle typify gentler gradients near the mouth.
Fluvial (river) sediments are called alluvium.
- Channels may have mid-channel bars
- Sands build up point bars inside channel bends.
- A stream builds a delta upon entering a lake/ocean.
Stream character changes with flow distance.
In profile, gradient is a concave-up curve.
Near stream headwaters...
-Gradient is steep, discharge is low.
- Sediment sizes are coarse (large).
- Channels are straight, rocky.
Toward the mouth (downstream end)...
-Gradient is low, higher discharges
- Smaller grain sizes typical.
- Channels are larger, bend more.
Base Level Concept
Lowest point to which a stream can erode.
-Ultimate base level is sea level.
--- Streams cannot erode below sea level.
- A lake serves as a local (or temporary) base level.
- Base level changes cause stream to adjust.
---Raising base levle results in an increase in deposition.
--- Lowering base level accelerate erosion.
Valleys and Canyons
Land far above base level is subject to down cutting.
Rapid down cutting creates eroded trough.
- Valley- Gently sloping trough sidewalls define a V-shape.
- Canyon- Steep trough sidewalls form cliffs.
Determine by rate of erosion vs strength of rocks.
Valleys store sediment when base level is stable/raised.
Stability, then renewed incision creates stream terraces.
---Terraces are former, now abandoned, floodplains.
Rapids & Waterfalls
Rapids are turbulent water with a rough surface.
Waterfalls are free-falling water columns.
Reflect geologic control:
-- Flow over bedrock steps or large clasts.
-- Flow constriction (channel narrowing)
-- Sudden increase in gradient.
Build at mountain front by river (or debris) flow.
Sediments rapidly dropped near stream source.
Sediments create a conical, fan-shaped structure.
Form where channels are choked by sediment.
Flow is forced around sediment obstructions.
- Diverging- converging flow creates sand & gravel bars.
- Bars are unstable, rapidly formed and eroded.
Flow occupies multiple channels across a valley.
Channels can form looping curves.
- Along lower river portion with low gradient.
- Where streams travel over a broad floodplain.
- When substrates are soft & easily eroded.
Meanders increase volume of water in the stream.
Max velocity swings back & forth across channel.
-Fast water erodes cut bank (outside of bend).
- Point bar (inside of bend).
Meanders change due to natural variation in...
- Thalweg (maximum depth) position & friction.
- Get cutoff when sinuosity gets too severe (cut banks coverage).
Meanders become more sinuous with time.
- Cut bank erodes; point bar accretes.
- Curves become more pronounced.
Deltas form --> a river enters
- Flow slows, loses competence; sediments drop out.
Channel divides into a fan of small distributaries.
Mississippi has a river-dominated bird's foot delta.
Distinct lobes indicate past deposition centers.
River periodically siwtches course via avulsion.
- River breaks through a levee upstream.
- Establishes a shorter, steeper path to the Gulf of Mexico.
Streamflow is cause of most landscape changes.
- Uplift changes base level.
- streams cut down.
- Valleys widen; hills erode.
- Landscape lowered to new base level.
- One stream captures flow from another.
- Results from headward erosion.
- A stream with more vigorous erosion ( steeper gradient) intercepts another stream.
- Captured stream flows into the new stream.
- Below capture point, old stream dries up.
-Tectonic uplift can alter a river course.
- South America used to drain westward.
- Wester uplift raised the Andes, change Amazon flow to east.
- Tectonic uplift can raise ground beneath established streams.
- If erosion keeps pace with uplift, stream will incise into uplift.
-- Called antecedent drainage.
-If uplift rate exceeds incision, stream is divered around uplift.
Some antecedent strams have incised meanders.
- Meanders initially develop on a low gradient.
- Uplift raises landscape (drops base level)
- Meanders incise into the uplifted landscape.
During a flood.
- Flow exceeds water volume storage of a channel.
- Velocity ( thus, competence & capacity) increase.
- Water leaves channel, drowns adjacent land.
- Moving water & debris scour floodplains.
- Water slows away form the thalweg, dropping sediment.
Numerous causes of floods:
- torrential rainfall
- After soil pores have been filled by prior rarinfalls.
- abrupt warm weather rapidly melts winter slow.
- Failure of a natural/artificial depth.
Case history: Mississippi and Missouri Rivers, 1993."
• Spring 1993:"
• July 1993:"
• Covered 40,000 mi2."
• Flood lasted 79 days."
• 50 people died."
• 55,000 homes destroyed."
• $12 billion in damage."
Seasonal floods recur on an annual basis.
- Monsoons- heavy tropical rains (ie on Indian subcontinent)
-- Intense period of heavy summer rain
-- Many people live in floodplain & delta plain settings.
---1990: monsoon killed 100,000 people in Bangladesh.
---2008- Monsoon caused the Kosi river to avulse, displacing ~2.3 million people in Nepal/India.
•␣ Kosi River flood before and after.␣
•␣ New channel width ~20 km!␣
Ancient floods: Ice-Age megafloods.
- 11 Ka, ice dams failed, releasing Glacial Lake Missoula.
- Water scoured eastern Washington landscape.
---Created "channeled scablands".
--- One of the largest floods in geologic history!
Living with Floods
People living in floodplains face hard choices.
-Move or expect eventual catstrophic loss.
Land use changes may mitigate flood damage.
- Establish floodways- places designed to transmit floods.
-Remove people & structures from these places.
Living with Floods
Flood risk borne by homeowners, insurance companies, lenders, government agencies.
- Use hydrologic data to produce flood risk maps.
- Maps allow agencies to manage risks.
- Building in flood-prone settings is tightly regulated.
Living with Floods
Flood risk is calculated as a probability.
- Discharges are plotted against recurrence intervals.
- On semi-log, this plots as a straight line.
- Probability (% chance of occurence) given discharge will happen (determined by graph inspection.