Seasonal variation in discharge defines river regime. Three broad classes of regime can be distinguished for perennial streams. In the megathermal class, related to hot equatorial and tropical climates, two main variants occur; discharge is powerfully sustained throughout the year, usually with a double maximum (two peak values), but in some areas with a strong single maximum. In the mesothermal class some regimes resemble those of tropical and equatorial areas, with single or double summer maxima corresponding to heavy seasonal rainfall, while others include sustained flow with slight warm-season minima. Where midlatitude climates include dry summers, streamflow decreases markedly and ...(100 of 32811 words)