Cambridge IGCSE Geography

Rivers and drainage basins

A drainage basin is the area of land that a river and its tributaries drain. Rain that falls anywhere in the basin ends up, one way or another, in the same main river. The edge of the basin is the watershed, a line of high ground (often a ridge) beyond which water drains into a different river.

Discharge is calculated from the size of the channel and the speed of the water:

The cross-sectional area is the width multiplied by the average depth, so a channel 8 m wide and 1.5 m deep (average) has an area of $8 \times 1.5 = 12\,\mathrm{m^2}$. If the water moves at $0.5\,\mathrm{m\,s^{-1}}$, the discharge is $12 \times 0.5 = 6\,\mathrm{m^3\,s^{-1}}$.

The wetted perimeter matters because water in contact with the bed and banks is slowed by friction. A channel with a small wetted perimeter for its area (a deep, semicircular channel) loses little energy to friction, and a channel with a large wetted perimeter (a wide, shallow channel) loses a lot. The ratio of cross-sectional area to wetted perimeter is called the hydraulic radius, and a higher value means a more efficient channel and faster flow.

The long profile and the Bradshaw model

The long profile of a river is a graph of its height against distance from the source to the mouth. It is a smooth, concave curve: steep near the source and gently sloping near the mouth. The cross profile is the shape of the valley and channel at one point along the river.

The Bradshaw model describes how a river's characteristics usually change from its upper course to its lower course. It is a generalisation, and real rivers vary because rock type, tributaries and human structures such as dams change the pattern.

It surprises many students that velocity increases downstream even though the gradient falls. The reason is friction. In the upper course the water is shallow and tumbles over large boulders, so a large share of its energy is lost as turbulence and friction with the bed. Downstream the channel is deeper and smoother, the wetted perimeter is small compared with the volume of water, and so the water flows faster despite the gentler slope.

The drainage basin in the water cycle

The water cycle is the continuous movement of water between the oceans, the atmosphere and the land. A drainage basin is an open system: it has inputs, stores, flows (transfers) and outputs, and it exchanges water with its surroundings.

The input is precipitation (rain, snow, hail and sleet). The outputs are the river's discharge to the sea and the water lost to the atmosphere by evaporation and transpiration. Between them, water is held in stores such as vegetation, surface puddles, soil, rock and the river channel itself.

Processes in the basin

• Precipitation is water falling from the atmosphere.
• Interception is rainwater caught on the leaves and stems of plants (and on buildings) before it reaches the ground. Some evaporates straight from the surface and some drips to the ground later, so a forest delays and reduces the amount of water reaching the soil.
• Infiltration is the movement of water from the surface into the soil. Its rate depends on how wet and compacted the soil is, how steep the slope is and whether the surface is bare, vegetated or paved.
• Percolation is the downward movement of water from the soil into the rock below, where it becomes groundwater.
• Overland flow (surface runoff) is water flowing over the surface, which happens when the ground is saturated, frozen, baked hard or paved, or when rain falls faster than it can infiltrate. It is the fastest route to a river.
• Throughflow is water moving sideways through the soil, slower than overland flow because the water has to squeeze through the soil pores.
• Groundwater flow is the very slow movement of water through permeable rock below the water table, which keeps rivers flowing in dry weather.
• Channel flow is the water travelling in the river itself.
• Evaporation is the change of liquid water into vapour from surfaces such as the sea, lakes, rivers and wet ground.
• Transpiration is the loss of water vapour from the pores of plant leaves.
• Evapotranspiration is evaporation and transpiration taken together, and it is usually measured as a single loss from a vegetated area.

The faster water gets to the river, the higher and earlier the peak in discharge after a storm. Overland flow gives a quick, high peak, throughflow a slower one, and groundwater flow a very slow, steady supply.

What changes the speed of the response

Several features of a basin affect how fast rainfall becomes river flow. Impermeable rock (clay, granite) and paved surfaces force water to run off the surface, while permeable rock (chalk, sandstone) lets it soak in. Steep slopes shed water quickly, and gentle ones hold it longer. A round basin delivers water from every side to the same point at about the same time, so it produces a higher flood peak than a long, narrow basin, where the water arrives in sequence. Dense vegetation intercepts rain and slows the water, and bare or ploughed ground does the opposite. Saturated, frozen or very dry and baked soil all infiltrate poorly.