Climate, described here using the Köppen–Geiger classification, plays a dominant role in determining the total flow, interannual variability, seasonal regime, extremes of high and low flow of rivers, and the nature of river channels. Mean annual runoff is the product of the climatic water balance which in arid areas is dominated by losses to evapotranspiration and the runoff component increases as climate becomes more humid. Arid and semiarid regions with little runoff occupy nearly one third of the continents. Below about 600 mm of annual precipitation, there is a strong negative correlation between annual precipitation and the variability of annual runoff. The distribution of runoff through the year is referred to as the seasonal regime and is determined by the pattern of precipitation and temperature throughout the year. Extremes of flow – floods and low flows – and their time of occurrence through the year are also a product of climate characteristics. River channels in arid climates are formed by isolated extreme events, while in humid climates they are in equilibrium with the range of flows that do most work because of their particular combination of frequency and magnitude. Areic drainage systems occur where there is no integrated channel network – most commonly in arid areas; endoreic drainage systems are confined to topographic basins and do not flow to the sea; exoreic drainage systems flow from the continents into the oceans and occupy two-thirds of the continental area. The regulation of flow in rivers by water extraction, storage in dams and interbasin transfers now affects most major river systems on the planet with consequent impacts on instream aquatic ecosystems.