Test. Name and explain the 4 ways that a river transports its load? Name and explain 5 ways that the river erodes? Give three reasons why a river deposits?. Rivers. Drainage Patterns
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Name and explain the 4 ways that a river transports its load?
Name and explain 5 ways that the river erodes?
Give three reasons why a river deposits?
Dendritic: Tree like branch pattern where tributaries converge into a main river. Develops on gently sloping land of uniform rock type. E.g. Shannon
Radial: Streams move out from a central high point - a hill or mt. Wicklow mts.
Trellised: Tributaries join at right angles. Occurs where land has been folded or varied rock bands. E.g.. Lee
Deranged: Random and chaotic pattern. Found in marshy lowlands & clay soils.
E.g.. Cavan area.
River Discharge: Volume of water passing a certain point at a given time in a river channel. Greater the discharge = great capacity to erode & transport.
Discharge greater in winter.
Shown on a hydrograph…. Shows suitability for HEP, water supply & irrigation. Also used to predict flood risk & flood control measures.
River Velocity: (speed) Effected by 1. Gradient… steeper = faster
2. Discharge… larger = more energy
3. Channel shape… more contact with bed & banks (wetted perimeter) - more friction - reduce energy & speed - 95% of energy used for moving
4. Channel roughness… uneven bed & banks, potholes & boulders - friction & turbulent water - reduce energy & speed
River Flow: Mainly turbulent flow, this increases with velocity-- movement in all directions – necessary for transportation and erosion
Processes of Erosion
Hydraulic action: The force of the water….. Stronger current = greater erosion.
turbulent and eddying water can undermine banks causing bank caving. Cavitation (air bubbles collapsing forming tiny shock waves also loosen soil.
Solution: Chemical erosion of rocks, dissolving minerals which are carried in solution, particularly in areas of chalk or limestone
Abrasion: River armed with material (pebbles, sand, gravel) which it uses to erode. Uses it load to erode.Greatest erosive power when in flood. Pebbles whirled by eddies form potholes.
Attrition: River load is worn down as it hits off itself, pebbles are rounded and smooth
Vertical erosion: Downward erosion of the bed
Lateral erosion: Side wards, erodes the bands
Headward erosion: River erodes in the opposite direction to which it flows.
Processes of Transportation
Solution: The dissolved load.
Suspension: Visible cloud of sediment (brown colour) -- clay & silt carried within the flow.
Saltation: Smaller particles are bounced along the river bed subject to the force of the water.
Traction: Heavy load is rolled along the river bed.
Material sorted by deposition… heaviest dropped first then lightest (clay & silt) last = alluvium.
Depends occurs when:
1. Velocity… reduced as it enters sea or lesser gradient.
2. Discharge…. Less water due to less precipitation
3. Increased load…. Increased soil erosion after a flood or if new tributary joins
Waterfall: A layer of resistant rock lies beside or across soft rock. Soft rock erodes faster --differential erosion - a fall develops as the river bed is steepened. Water drops on the underlying soft rock and wears it away.
Water backlashes and mist rots rock to form a cave. The overhang collapses since the rock is undercut. This collapsed rock swirls & carves out a plunge pool.
Hydraulic action: Speed is increased as it approached the fall - increase hydraulic action
Attrition: Eroded the soft rock and deepened the plunge pool. It dissolves, weakens & breaks up rocks.
Abrasion: Eroded material used to gouge out a plunge pool.
Waterfall retreats upstream -- headward erosion creating a gorge
Niagra falls has created a 12km gorge, retreating upstream by 1m a year. Has been reduced as some water now diverted for HEP
E.g. Powerscourt Co Wicklow. Dargle.
Torc, Killarney Co Kerry. Flesk
Meander & ox-bow lake
Meanders are curves or bends in a river. Formed by erosion & deposition.
Water flows faster on the concave (outside) & slower on the convex (inside). The outside bend increases in size due to hydraulic action & abrasion. This lateral erosion increases the curve size.
Secondary flows (corkscrew in shape) move material from the convex & deposit it on the concave. This deposition of sand & shingle forms a point bar.
The meander becomes pronounced & a neck develops. During a flood the rivers cuts through he neck & forms a new course.
In time the river deposits along its banks & cuts off the bend forming an ox-bow lake and eventually a meander scar.
E.g.. Nore Co Kilkenny, Suck, Co Galway.
A flat low-lying area flooded by a river.
Processes: Corrasion, attrition, solution.
Lateral erosion predominates. River floods, overflows its banks & deposits silt. Coarser material deposited close to banks & alluvium (finer) deposited on outer flood plain. Edge of flood plain marked by a bluff line (slope). Material accumulates over time. Forms levees on river banks. Drained flood plains are very fertile…. good agric. land.
E.g.. Shannon, south of Athlone.
Delta: A flat area of alluvium deposited by a river when it enters a lake or the sea. Velocity of river decreases and it deposits its load. Flocculation is where sediments clot on contact with sea water.
Heavier material is laid down first & the lighter material carried further. The tides are unable to remove the sediments and the river channel becomes choked.
Bottomset Beds are layers of fine sediment carried out furthest
Foreset Beds are thicker coarser layers that are laid closer to the river
Topset beds are a mixture of material deposited nearest to the land
These form islands or braids. This causes the mouth of the river to divide into channels called distributaries.
Knickpoint: The intersection between the old and new profile. Often marked by a waterfall or weir. Due to headward erosion.
EG Barrow, Bagenalstown Co Carlow.
Paired Terraces: Vertical erosion abrasion & hydraulic action erodes the clay silt and bedrock and forms a new narrower floodplain at a lower level. Lateral erosion creates a pair of terraces as margins of the floodplain. A series of these occurs with rejuvenation & are left high above the new floodplain.
EG Dodder, Terenure, Dublin
Incised meander: If downcutting is severe the river cuts a deep meander into the valley floor.
EG Nore, Thomastown Co Kilkenny,
Drainage pattern of south of Ireland
A chalk surface was uplifted & faced south.
Consequent streams…. Follow direction of slope, flowed south
Eroded and exposed sandstone anticlines. Rivers cut into landscape and superimposed themselves.
Subsequent streams… tributaries flowed at right angles to the consequents. Developed as they flowed on the chalk. These streams, by headward erosion, cut through to new catchment areas. They intercepted the consequent & changed its direction of flow. This is called river capture.
A beheaded stream remains & the bend where the river was captured is called the elbow of capture. Below this a wind gap marks the now dry former course of the beheaded stream. The beheaded stream is a misfit stream as it is very small in relation to the valley through which it flows.
The chalk was eroded & tributaries flowed down the anticline, joining at right angles. This gives us a trellised pattern of drainage along the Suir & Blackwater rivers.