Define the term drainage basin., An area of land drained by a river and its tributaries., Explain how a watershed separates two drainage basins., It is a highland boundary that separates the drainage areas of two river systems., Explain how vegetation affects the drainage basin system., Vegetation intercepts rainfall, increases infiltration and can reduce surface runoff., Explain how infiltration affects river discharge., Greater infiltration allows more water to enter the soil, reducing rapid surface runoff and lowering peak discharge., Explain how soil saturation can increase surface runoff., Once the soil becomes saturated, it cannot absorb much more water, so additional rainfall flows over the surface., Describe the relationship between rainfall and river discharge., Increased rainfall usually increases river discharge, although the response depends on factors such as soil, vegetation, slope and drainage-basin size., What is lag time?, The time between peak rainfall and peak river discharge., Explain why a steep rising limb occurs on a storm hydrograph., Water reaches the river quickly because of rapid runoff, limited infiltration or impermeable surfaces., Explain how deforestation can affect river discharge., It reduces interception and infiltration, increasing surface runoff and potentially increasing peak discharge., Explain how urbanisation can increase flood risk., Buildings and roads create impermeable surfaces, reducing infiltration and increasing rapid surface runoff., Explain how hydraulic action erodes a river channel., The force and pressure of moving water loosen and remove material from the river bed and banks., Explain how abrasion changes a river channel., Sediment carried by the river strikes and scrapes the bed and banks, causing erosion., Explain how attrition affects a river's load., Rocks collide during transportation, causing them to become smaller, smoother and more rounded., Explain why deposition occurs., A river loses energy and can no longer transport some or all of its load., Explain how a meander develops., Faster flow causes erosion on the outer bend while slower flow causes deposition on the inner bend, increasing the size of the bend., Explain how an oxbow lake forms., Erosion narrows the meander neck until the river cuts through it; deposition then seals off the old meander loop., Explain how a waterfall retreats upstream., Erosion undercuts softer rock, causing the harder rock above to collapse. Repeated collapse causes the waterfall to retreat upstream., Explain how a floodplain forms., Repeated flooding deposits alluvium across the valley floor, gradually creating a wide, flat floodplain., Explain how levees form., During floods, heavier sediment is deposited close to the river channel, gradually building raised banks., Explain one impact of dams on river discharge., Dams can regulate the amount of water released downstream, reducing variations in discharge., Explain one environmental impact of large dams., They can flood habitats, alter ecosystems and interrupt the natural movement of sediment., Explain why the Ganges-Brahmaputra-Meghna basin is vulnerable to flooding., Bangladesh is low-lying, receives heavy monsoon rainfall and receives water from large rivers draining the Himalayas., Explain how monsoon rainfall affects flooding in Bangladesh., Heavy seasonal rainfall rapidly increases river discharge and can cause widespread flooding., Explain why the Amazon Basin has a large annual water flow., It has high rainfall, numerous tributaries and a large drainage basin located mainly in a humid tropical environment., Explain one major impact of deforestation in the Amazon Basin., It reduces interception and can increase surface runoff, soil erosion and river discharge., Explain why the Colorado River faces water-supply problems., High demand from agriculture, cities and industry places pressure on limited and variable water supplies., Explain one advantage of the Thames Barrier., It protects low-lying areas of London from tidal flooding and storm surges., Explain one disadvantage of hard-engineering flood management., It can be expensive to construct and maintain and may transfer flood risk to other areas., Explain how afforestation can reduce flood risk., Trees intercept rainfall, increase infiltration and slow surface runoff, reducing the amount of water reaching rivers quickly., Explain why wetlands are useful in flood management., Wetlands store excess water and release it slowly, reducing peak river discharge and flood risk.

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