ESS FINAL PREP: RIVERS

how do rivers shape landscape?

erosion, transport,+deposition, carving valleys, building floodplains, +forming deltas.

E’s surface water makeup?

Oceans (96.5%), OF NON OCEAN %: glaciers+ice sheets (68.6%), Groundwater (30.1%), freshwater 1.3% —> snow and ice (73.1%), lakes (20.1%).

what is hydrologic balance?

over oceans —> evaporation exceeds precipitation, over land —> precipitation exceeds evaporation.

Balance is maintained by runoff returning water to the oceans (36,000 km³/yr)

what does the amount of runoff depend on?

the amount of runoff depends on: rainfall intensity&duration, soil saturation, surface material (impermeable vs permeable), land slope, vegetation cover.

High runoff occurs when: surface material is impermeable or saturated.

what is the runoff progression?

rain —> sheet flow —> rill —> gulley —> ravine —> stream valley —> river valley —> gorge —> canyon

drainage basin (watershed)

def: the land area drained by a stream + its tributaries.

Divide: the boundary separating drainage basins, visible as a ridge in mountainous areas or subtle in flatter regions.

Outlet: the point where a stream exits the drainage basin, typically at a lower elevation. DB can be any size

what is hillslope erosion?

rain impact+sheet flow move sediment downslope into streams. gradually wears down slopes, contributes fine particles to waterways.

*major source of sediment transport in river systems.

what is headward erosion?

streams lengthen by cutting channels deeper at their source. extends valleys into previously undisected terrain, occurs when flowing water gains enough energy to erode upstream

3 Zones of River System. 1) sediment production

def: occurs in the headwaters, where erosion is most active. Sources: weathered bedrock, mass movement, bank erosion, + scouring of the channel bed

3 Zones of River System 2) sediment transport

def: takes place in the trunk streams, where sediment is moved but not stored. Transport mechanisms: suspended load, bed load, + dissolved load

3 Zones of River Systems 3) sediment deposition

def: happens when a river slows, reducing its ability to carry sediment. Deposition sites: river mouth, floodplains, alluvial fans, costal areas.

how does dendric (tree like pattern) form?

forms on uniform material wiht consistent resistance to erosion

where is radial (drainage) pattern common?

around volcanic cones and domal uplifts

rectangular (drainage) patterns form where?

develops in bedrock crisscrossed by faults or joints

trellis (drainage) pattern forms where?

forms in regions with alt bands of resistant+less-resistant rock (Appalacian mtns)

what are antecedent streams?

pre-existing streams that cut through rising terrain by eroding downward

what is laminar flow?

def: water moves smoothly in parallel layers, common in very slow moving streams

what does turbulent flow mean?

def: water moves chaotically, forming whirlpools, eddies +whitewater rapids. - increases erosion

factors affecting flow velocity

gradient/slope: steeper gradient=greater gravitational energy driving streamflow.

channel shape: cross-sectoinal shape determines how much water is in contact with the bed+banks (wetted perimeter: wide, shallow channels have more friction+lower velocity).

Channel size/roughness: larger channels allow faster flow (less friction).

Rough channels increase turbulence +slow flow.

what is Longitudinal Profile?

def: cross-sectional view of a stream from its headwater to its mouth. Typically shows a smooth, concave curve w/ a decreasing gradient downstream.

headwaters to mouth

Gradient (slope): steep in the headwaters, gradually decreasing downstream+decreasing downstream.

2. Discharge (Water Volume): increases as tributaries contribute more water to the main channel.

3. Channel Size: becomes wider+deeper downstream to accommodate increased discharge (less friction).

4. Sediment size+transport: coarser sediment dominates in headwater. Finer sediment is more common downstream.

what are the categories in Transport of Sediment Load ?

1. Dissolved Load (soluble material carried in solution, independent of velocity.

2. Suspended load: fine sediment (silt, clay) carried within the water column.

3. Bed load: coarse sediment moves along the bottom by rolling, sliding, or saltation (def:changes)

what are the factors in sediment transport?

1. Capacity: max load a stream can carry (related to discharge)

2. Competence: max particle size a stream can transport (related to velocity)

3. Settling Velocity: speech at which particles fall through still water; larger, denser, + more spherical particles settle faster.

when does Deposition of Sediment happen?

when velocity decreasing, reducing competence.

Larger particles settle first - resulting in sorting of sediment by size.

what do meandering channels do?

sweeping bends, carry fine sediment in suspension, erosion occurs at the outer curve (cut bank) while deposition forms point bars on the inner curve, meanders migrate over time

what are Braided Channels?

def: multiple INTERWOVEN CHANNELS separated by gravel bars. Form where sediment load is high+discharge varies greatly

Channels shify laterally as sediment is eroded, transported, + redeposited

what is Base Level?

def: lowest point of a stream can erode to, controlling how much downcutting occurs.

Changes in base level: rise (dam construction, rising sea levels)- leads to sediment deposition. Drop in base level (sea level fall a/o uplift)- increases downcutting+valley deepening

what are Graded streams?

def: streams that have adjusted their slope+flow to achieve balance between erosion+deposition.

when do antecedent streams occur?

occur when uplift is gradual enough for the stream to maintain its course

what is deposited sediment called?

alluvium

where are braided channels common?

in glacial meltwater streams where seasonal discharge shifts sediment.

what is ultimate base level?

Ultimate base level: sea level, the lowest elevation a stream can reach.

what is local base level?

temporary stopping points for erosion, such as lakes, resistant rock layers or trunk streams.

what is dynamic equilibrium?

neither actively eroding nor depositing, just transporting sediment.

what are some disruptions of equilibrium?

inc sediment load→ deposition inc, raising channel bed. Inc discharge→ erosion intensifies, deepening channel.