You are expected to be able to construct, describe and explain hydrographs. A hydrograph shows two graphs – rainfall (in bars) and discharge (in a line).
The peak rainfall is the time of highest rainfall. The peak discharge (the time when the river reaches its highest flow) is later because it takes time for the water to find its way to the river (lag time) . The normal (base) flow of the river starts to rise (rising limb) when run-off, ground and soil water reaches the river. Rock type, vegetation, slope and situation (ie is this an urban river?) affect the steepness of this limb. The falling limb shows that water is still reaching the river but in decreasing amounts. The run-off/discharge of the river is measured in cumecs – this stands for cubic metres per second. Precipitation is measured in mm – this stands for millimetres.
Why does a river flood?
You should be able to look at hydrographs (sometimes with a map as well) and explain why one river floods and another does not. Sometimes you will be given one river’s hydrograph over a series of days like the one below.
Hydrograph – Rain storms
You are expected to be able to describe and explain the formation of various river features. These features can come from the upper, middle and lower course of a river. A popular but poorly explained feature is the meander, so we will feature it here, but you should learn more than one feature!
Formation of a meander
Water never flows in a straight line even in an apparently straight river channel. Water twists and turns around stones and other obstructions resulting in areas of slower and faster water movement. Slower areas are found in deeper parts of the river filled with fine sediments and are called pools. Faster areas are found in shallower parts of the river around larger stones and are called riffles.
How a river changes shape
The river starts to flow from side to side in a winding course but still in a relatively straight channel. The pools tend to move to opposite sides of the channel over time and this is where a meander will develop.
Water moving faster has more energy to erode. This occurs on the outside of the pool.
Revise the types of erosion the river water can carry out – attrition, corrasion, corrosion and hydraulic action.
Cross-section of a river channel
Water moving slowly tends to deposit material as it has little energy left to erode or transport material. This occurs on the inside of the pool.
Revise the types of transportation the river can do – traction, saltation, suspension and solution.
The changing shape of a meander
Meanders migrate downstream and change shape over time. The neck of land between the loops also gets narrower. At the next flood, the river may cut through this to find a faster course. It then deposits material at the entrance to the old meander, cutting off the flow of water and creating an ox-bow lake.