2.2.2 River processes

Terminology for the study of rivers

• ·       Drainage basin - the area of land drained by a river.
• ·       Catchment area - the area within the drainage basin.
• ·       Watershed - the edge of highland surrounding a drainage basin. It marks the boundary between two drainage basins.
• ·       Source - The beginning or start of a river.
• ·       Confluence - the point at which two rivers or streams join.
• ·       Tributary - a stream or smaller river which joins a larger stream or river.
• ·       Mouth - the point where the river comes to the end, usually when entering a sea.

Changes in a river from top to bottom:

Drainage basins act as a system with inputs, (precipitation) transfers (stemflow, infiltration, percolation, surface runoff, throughflow, & groundwater flow) stores, (interception, surface storage, soil moisture storage & groundwater storage) and outputs (evaporation & transpiration or evapotranspiration)

Transportation – river transports material by 4 processes

•             Traction – rolling stones along the bed
•             Saltation – sand-sized particles bounce along the bed in a leapfrog motion
•             Suspension –silt and clay-sized are carried within the water flow
•             Solution – minerals dissolve in the water

Erosion – the wearing of land and rocks, there are 4 types

• Attrition – large particles such as boulders collide and break into smaller pieces (occurs at higher part of river)
• Hydraulic action – the sheer force of the river dislodges particles from its banks and bed
• Corrasion – smaller particles rub against the river banks and bed like sand-paper; also called abrasion; occurs at low part of river (where there are smaller particles)
• Corrosion – acids in the river dissolve rocks (occurs at any part of river)

Deposition – when a river lacks the energy to carry its load; it begins with the heaviest particles; happens when there is less water (a dry spell) or where the current slows down (e.g. the inside of a meander). Large boulders are deposited at the top, and very small particles are deposited at the end, resulting in sorting.

Associated landforms:

• V-shaped valleys: Near its source, a river is high above sea level so most of the erosion is vertical.

• In theory this would produce vertical valley sides, but once exposed the valley sides are weathered. The loose material falls down the slope & is carried away by the river, resulting in a steep sided V-shape.

• Interlocking spurs: In the upper valley a river is in the mountains. Water takes the easiest path downhill so twists & turns around the high land (spurs) forming interlocking spurs.

• Waterfalls: They occur because the river flows over hard rock which erodes slowly.

• Beneath is softer rock which is eroded faster to form a “step”.

• The force of the water erodes the bottom of the waterfall to form a plunge pool.

• The hard rock gets undercut as the soft rock erodes so that it eventually collapses.

• Rapids: They form also where the river passes over hard rock, but either the band of rock is not very deep or there are a series of shallow rock bands.

• Potholes: Can be found in the upper & middle valley where a river flows over solid rock.

• Swirling water forms eddies which can cause stones to move in circular motions so eroding circular holes in the rock.

• Meanders: Wide sweeping bends found in the lower part of the river.

• They are formed by a combination of lateral erosion & deposition.

• They help form the flood plain.

 

• Ox-bow Lakes: Ox-bow lakes form when the neck of the meander becomes very narrow.

• During high flow or floods the river cuts through the neck & straightens its course.

• Deposition occurs on the bank of the river

• The cut-off meander is an ox-bow lake.

• Delta: Form when a large river is carrying a high sediment load. Ex: Nile Delta. The river loses energy as it enters a lake or sea. The sea is sheltered and has few currents to carry away the sediment so it is deposited in the mouth.

Sediment is deposited and chokes up the channel forming islands. The river breaks up around these deposits forming several channels called distributaries.

•Flood plain: The river widens its valley by lateral erosion. At times of high discharge, the river has plenty of energy so carries a lot material in suspension. When the river floods, the water spreads across the flat land. The sudden increase in friction will reduce the water’s velocity and the fine silt is deposited. Each time the river floods, it deposits a layer of fine silt. This is a flood plain.

•Levées: when a river floods, the coarsest material is deposited first (so on the edges of the river) forming a natural embankment called a levée.

Causes of flooding:

• ·       A steep-sided channel - a river channel surrounded by steep slopes causes fast surface run-off.
• ·       A lack of vegetation or woodland - trees and plants intercept precipitation (ie they catch or drink water). If there is little vegetation in the drainage basin then surface run-off will be high.
• ·       A drainage basin, consisting of mainly impermeable rock - this will mean that water cannot percolate through the rock layer, and so will run faster over the surface.
• ·       A drainage basin in an urban area - these consist largely of impermeable concrete, which encourages overland flow. Drains and sewers take water quickly and directly to the river channel. Houses with sloping roofs further increase the amount of run-off.
• ·       Deforestation, overgrazing and overcultivation, and population pressures cause soil erosion causes sediment to go into rivers decreasing the cross-sectional area

Flood management: afforestation, dams, canalising rivers, artificial levées, flood relief canals, warning systems, dredging, cutting off meanders to straighten river, flood gates

Dams:

•      Dams are often built along the course of a river in order to control the amount of discharge. Water is held back by the dam and released in a controlled way. This controls flooding.
•        Water is usually stored in a reservoir behind the dam. This water can then be used to generate hydroelectric power or for recreation purposes.
•        Building a dam can be very expensive.
•        Sediment is often trapped behind the wall of the dam, leading to erosion further downstream.
•       Settlements and agricultural land may be lost when the river valley is flooded to form a reservoir.
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River engineering:

•  The river channel may be widened or deepened allowing it to carry more water. A river channel may be straightened so that water can travel faster along the course. The channel course of the river can also be altered, diverting floodwaters away from settlements.
•   Altering the river channel may lead to a greater risk of flooding downstream, as the water is carried there faster.

Afforestation:

Trees are planted near to the river. This means greater interception of rainwater and lower river discharge. This is a relatively low cost option, which enhances the environmental quality of the drainage basin.

Managed flooding:

The river is allowed to flood naturally in places, to prevent flooding in other areas - for example, near settlements.

Planning:

•     Local authorities and the national government introduce policies to control urban development close to or on the floodplain. This reduces the chance of flooding and the risk of damage to property.
•        There can be resistance to development restrictions in areas where there is a shortage of housing. Enforcing planning regulations and controls may be harder in LEDCs.

Hydrographs: