The lag time between peak discharge and peak precipitation reflects the time required for water to move down slopes and through progressively larger stream channels. In humid regions, annual stream hydrographs show discharge peaks from individual rainfall events superimposed on base flow derived from groundwater. Floods occur when discharge exceeds the capacity of a river channel and water flows over the low-lying floodplain adjacent to the channel. Lakes are important elements of the drainage system because they are sources of fresh water, and are used for recreation and hydroelectric power generation. Lakes without a surface outlet often become saline as they lose water primarily through evaporation.
Human activities can cause pollution of both surface and underground water.
Draw a simple cross-section diagram of a valley. Show the likely configuration of the water table and the flow paths of the groundwater on your diagram. Give an example. Sketch a hydrograph to show how stream flow responds to a rainfall event. Indicate base flow on your diagram.