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Small Basins: In general, total runoff is proportional to drainage area, but the maximum flood discharge per square mile is less for large than for small basins. This is explained by the fact that storms are small in comparison with the size of large river basins.
Obviously, the dimensions of the water cross section in a river channel at a given place depend on the amount of water flowing in it. Maximum size is attained during floods, and between floods streams are smaller or may even be dry.
Most large rivers empty into the ocean, but there are many streams in arid regions which drain into interior basins. The Humboldt River in Nevada, for example, discharges its waters into Carson Sink, where they evaporate. Nearly one fourth of the land area of the earth drains into interior basins rather than into the oceans. Most of those basins are in arid portions of Asia, Africa, and Australia.
The major drainage regions delineated by the government agencies for study are: (1) The North Atlantic basins. This region extends from Pennsylvania to Maine and includes many relatively short river systems. (2) The South Atlantic and East-em Gulf basins.
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