What you need to know about rivers David

The River Helmsdale at Kildonan Photo: Michael Redfern

Essential definitions (1) Drainage basin Area of land drained by a river and its tributaries. Watershed Boundary between two drainage basins. Source Where a river starts. Mouth Where a river ends (usually at the sea or a lake). Tributary A smaller stream/river which flows into a larger one. Confluence The point where two rivers meet. Drainage density The length of all the rivers in a drainage basin, divided by the area of the drainage basin. The higher the drainage density, the greater the risk of flooding. Philip Allan Publishers © 2015

Essential definitions (2) Inputs Ways in which water enters the system through precipitation (rain, snow etc. ) Outputs Ways in which water is lost to the system, either when the rivers carry it to the sea or through evapotranspiration. Evapotranspiration The loss of moisture directly from water surfaces such as rivers and lakes (evaporation) or from vegetation (transpiration). Interception When trees, other plants etc. ‘interrupt’ the flow of water to the ground. Philip Allan Publishers © 2015

Essential definitions (3) Surface runoff Water flowing directly overland to the river (sometimes called overland flow). Infiltration Water passing through the earth surface inthe drainage basin into the soil layer. Throughflow The movement of water through the soil towards the river channel. Percolation The movement of water from the soil layer to the rock layer. Groundwater flow (sometimes called base flow) The movement of water through the rock layer towards the river channel. Philip Allan Publishers © 2015

The drainage basin hydrological cycle Philip Allan Publishers © 2015

Flood hydrographs (1) A flood or storm hydrograph shows how a river responds to one particular period of heavy rainfall. ‘Lag time’ is the time between the peak rainfall and the peak discharge of the river. Lag times can vary depending on the relief of the drainage basin, the underlying rock type, the vegetation, the land use and the drainage density. A river regime shows how the discharge of a river varies over a longer period of time — usually a year. Be aware of the factors that change the speed, and amount, of precipitation that reaches a river. Think of a river as system with: • Inputs • Stores • Transfers • Outputs Philip Allan Publishers © 2015

Flood hydrographs (2) This is a ‘flashy’ hydrograph with a short lag time showing that the river has risen quickly in response to heavy rainfall. On other occasions, the lag time may be longer, resulting in a ‘subdued hydrograph’ Philip Allan Publishers © 2015

Methods of erosion Hydraulic action Occurs when the sheer force of the water dislodges particles from the river beds and banks. Abrasion (also known as corrasion) Occurs when smaller material, carried in suspension, rubs against the banks of the river, wearing them away with a sand-papering action. Attrition Occurs when boulders and other materials being transported by the river collide and break up into smoother, smaller pieces. Corrosion (also known as solution) Occurs when acids in the water dissolve rocks such as limestone, which form the banks and bed of a river. Philip Allan Publishers © 2015

Methods of transportation Rivers pick up and carry material as they flow downstream. A river may transport material in four different ways: Traction Large boulders and rocks are rolled along the river bed. Saltation Small pebbles and stones are bounced along the river bed. Suspension Fine light material is carried along in the water. Solution Minerals are dissolved in the water. Deposition occurs when a river lacks the energy to carry its load — perhaps after a dry spell or on the inside of a meander where velocity is lower or where the river enters the sea. Philip Allan Publishers © 2015

Fine particles are The Hjulström curve cohesive and difficult Logarithmic scale (each cycle is a tenfold increase) to entrain Critical erosion velocity (CEV). At this speed river will entrain particles Sand is the first size to be entrained Once entrained particles can be transported at lower velocities than CEV Logarithmic scale showing wide range of Philip Allan Publishers © 2015 particle size Very fine-grained clays and muds are suspended in virtually still water Critical settling velocity. At this speed the river begins to deposit grains of differing sizes, coarsest first.

As you go downstream. . . There is an increase in: • Velocity • Discharge • Load amount • Cross-sectional area • Efficiency. There is a decrease in: • Gradient • Roughness • Friction • Turbulence • Load size Philip Allan Publishers © 2015

The upper course (1) Philip Allan Publishers © 2015

The upper course (2) In this stage: • The river is high above sea level and has lots of potential energy which it uses largely in vertical erosion. • The river valley is often V-shaped with interlocking spurs. • The channel is narrow and shallow with a large, angular bedload. • The channel has a steep gradient, especially at rapids and waterfalls, where the velocity of the water is relatively high. • However, the overall velocity is low as so much energy (up to 95%) is lost due to friction with the banks and beds. • The water is often very clear as there has been little abrasion and attrition — so the suspended load is very small. Philip Allan Publishers © 2015

The middle course (1) Philip Allan Publishers © 2015

The middle course (2) As a river flows downstream, the gradient becomes less steep and lateral (sideways) erosion becomes more important. The river then starts to meander. The flow is always faster on the outside bend of a meander. This means that the water has more power to erode its bed and so it is also deeper here. Meander migration starts. The water will also erode/undercut the river banks to form a steep-sided river cliff. On the inside bend, the water flows more slowly; the water is shallower and deposition of material will lead to the build up of a river beach (sometimes called a slip-off slope or point bar). Philip Allan Publishers © 2015

A meander on the River Lune in Cumbria Photo: Kevin Eaves/Fotolia Philip Allan Publishers

The lower course Philip Allan Publishers © 2015

The lower course In the lower course, the river becomes wider and deeper. The velocity also increases because there is less friction with the banks and bed. The bedload is smaller and more rounded as a result of the process of attrition. At this stage, the river will be carrying a large load of suspended material (brought from further upstream) and so deposition becomes the most important process. Philip Allan Publishers © 2015

Causes of flooding Physical Human Intense rainfall All linked to case studies Population growth Prolonged rainfall Urban growth Causes of flooding Snowmelt Deforestation Poor agricultural practices Impermeable rock Saturated ground

Physical/ environmental Human, social and economic Impacts of flooding All linked to case studies Casualties S o c Disease Damage to property/ dispossession Loss of crops/farm animals, food shortages Impacts of flooding i a l Overall cost/ Insurance Eco no Scale of flood Deposition of silt Meander cutoff/ levee breach mi c Infrastructure/ business damage Recharge groundwater stores

This resource is part of GEOGRAPHY REVIEW, a magazine written for A-level students by subject experts. To subscribe to the full magazine go to: http: //www. hoddereducation. co. uk/geographyreview Philip Allan Publishers © 2015