Rivers 1

Erosion definition

Wearing away of the land processes like ice, water and wind. River → lots of energy→ when velocity and discharge high

Hydraulic action def

Process of erosion caused by force of moving water. Force of water hits riverbanks - pushes water into crack, air compressed, pressure increases - riverbank can collapse, particles dislodged

Abrasion ( corrasion)

Bed and banks work away by material carried by river grinding/ scarping against them

Attrition

Load carried by river hits off other particles also carried by river. Collide fragments break off - overtime - stones smaller rounder

Corrosion (solution)

Weak acids in river water dissolve minerals from rock

Erosion faster

• Discharge high- period of rainfall , tributary adds water to main river channel

• Bedload small rounded - less friction less energy lost

• Steeper Gradient

• Wetted perimeter is small- I. E river narrow deep Bcus rock of bed is easily eroded - so small proportion of water in contact with bed and banks - small friction

• Human influences - removal of larger angular stones from riverbed - reduces friction straightening river channel increases gradient of riverbed making water flow faster

Transportation definition

Movement of water by the rivers flow

Size material relates to discharge, river velocity and discharge high. River has capacity to transport more Materials

Solution def

Transfer of Material in a dissolved state by the river

Sediment soluble e.g limestone or if river water - acidic has potential to dissolve the sediment

Suspension

Light particles lifted held, carried in flowing water

Normal flow - fine e.g clay silts

discharge high - more material large particles by suspension

Saltation def

When pebbles, sand, gravel on riverbed temporarily picked up by current and bounced along the river bed

-increased energy, discharge e.g after twin storm event particles too large for suspension

Traction def

When largest bedload (boulders, cobbles) roll or slide along riverbed , do not lose contact with it)

-sediment size exceeds prevailing energy level of river I.e river doesn’t haven enough energy to lift the particle off the riverbed

Deposition

When load river carrying has been transporting falls onto river bed as the river be no longer has enough energy to transport it

When does deposition occur

• Decrease river flow e.g after period of rainfall, building of a dam, not enough energy left to transport load

• River velocity is checked as it flows into sea/ lake

• River spills onto flood plain - friction with surface of floodplain - reduces river velocity

• Shallow water e.g inner bend/ bank of meander

Waterfalls formation

Upper course, form river flows over bands of rocks different resistances to erosion,

hard rock - little erosion , soft rock rock downstream eroded by hydraulic action, corassion solution.

Soft rock progressively steepens river bed until steep gradient waterfall formed.

Vertical gradient sheer force of water, and material in river soft rock eroded by hydraulic action and corassion to form plunge pool. Overlying hard cap rock resists erosion soft rock eroded quickly, hardcap is undercut. Hard cap rock weakened lack of support and weathering. Hard rock collapse fall into lunge pool. 

Material some remain in plunge pool eroding soft rock deeping plunge pool. Hard rock collapse waterfall retreats upstream creates a steep sided valley called gorge downstream of waterfall

Pools and Riffles 

form at regular intervals, river loses little energy as it flows over pool so erosion. Riffle - shallow depositstion occured river energy lower,  main river swings side to side avoid riffles. so energy and flow deflected to one side of river channel - erosion. Opposite side less flow energy so deposition.

Slight bends developed - meanders soon. River migrated laterally.

Meander formation 

(Middle -lower course) Outer bend - maximum river flow, deep, highest energy, eroded (hydraulic action corrasion) undercut. Outer bend undercut unstable collapse steep riverbank known as river cliff. Material eroded from outer bank moved downstream by helicoidal flow to next meander deposited at inner bank where shallow low energy, less in high loses of energy due to friction with bed and banks. River deposits material form point bar, erosion outer band deposition at inner bend, river channel migrates latterally and downsream

Ox-box lake formation

Crescnt shaped lake formed pronounced meander cut off from river flow. Outer bed erosion creates flow. Narrow neck of land seperates meander reduce in size then removed by erosion either graudally over long time or during high discharge high energy rnough to remove remaining narrow neck of land. Main river flow along straighter course, some river flow around meander much slower than straight so deposition happens along banks, deposition eventually cuts meander off to form ox bow lake.

Floodplain formation 

Lower course flat land on either side of river. RIver meanders around series of interlocking spurs, erosion (hydraulic action/corrasion) outer bend of slight meander - river flow divrted here, velocity high, cuts back interlocking spur.

Inner bend velocities slower, deposition occurs point bar begins to form. and increases in area.

Overtime meander migrates laterally and downstream. Interlocking spur cut back erosion, inner bank deposition point bar grows form area of flat land. Eventually river meander within a belt and low lying hills called bluffs on either side of floodplain, when river floods water slowed fiction with surface. Fine material river carrying deposited on floodplain, alluvium builds up floodpain further. 

Levees formation 

Natrually raised banks along river channel. When river velocity high at time of flood high energy transport huge load. River channel unable to hold discharge spill onto surrounding floodplain and slows due to friction with land surface.

Sudden loss of energy -depostion of largest and coarsest material first on banks, finest material transported by suspension to edge of floodplain as small aprticles can be transported low velocities. Repeated flooding builds up deposited sediment at banks of river form levees.

Conditions required for delta to form

1. River transport large load which reaches river mouth

2. Coastlines low wave energy + low tidal range, so material deposuted faster than erode by waves, currents, tides

3. Sea bed gentle graduebt so water shallow so deposited matieral build above sea level

4. Presence of salt warer - electrical charge cuases finest material to stick together in process- flocculation

5. Vegetation - establish community to stabilise it

Delta formation

River transporting huge load enters sea, velocity immediately checked, river begins depositing material carrying, leads to layers (topsets, - coarsest sediment above sea level, foresets- finer sediments, bottomsets - clay particles furthest out to sea suspension). Finest material deposited as electrical charge created by saltwater cause particles stick together- flocculation, further out sea before deposition.

Deposition at river mouth chokes channel to break up into smaller channels called distributaries, over time deposition along edge of distributaries as velocity low. Will grow in height above sea level if depo > eros. Area between distribs filled with sediment and rise above sea level  will be stabilised by vegeation 

Arcuate deltas

fan shaped - when coarser materials such as gravel depositied. areas where currents keep seaward edge of delta trimmed and smooth e/g Nile Delta, Egypt

bird foot deltas 

extend beyond coastline. formed from fine sediment, sea waves weak in comparison to rivers flow, sediment transported far from coastline before depo occurs e.g Mississippi Delta, USA