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001161 of highest elevation which enclose the area drained by each watercourse. Water- sheds of major rivers like the Connecti- cut or the Ohio are the sum of the sub- watersheds of tributary streams such as the Deerfield or the Allegheny. These in turn encompass still smaller watersheds of their tributaries. The entire land area of the U.S. therefore may be viewed as a complex jigsaw puzzle in which each "piece" represents a watershed which may in turn be divided into smaller "pieces," and so on. A map of water- sheds bears little resemblance to a map of political jurisdictions (Fig. 3). Watershoo& playa key role in the nat- ural process of flooding. The physical capacity of a stream channel in terms of width and depth is directly related to the average or normal flow of surface runoff reaching the stream from its watershed. But runoff in any watershed in fact varies greatly according to changes in precipita. tion, temperature, and other factors. At times of little nmoff, the stream channel can easily handle resulting flows. But when runoff increases above nonnal levels due to heavy rains or sudden snow. melt, the channel becomes inadequate. The action of moving water may scour the bottom and sides of the channel and increase its capacity somewhat, but when streamflow is much greater than nonnal, overbank flooding results. 3. What Is a Floodplain? In a geological time scale, floods are routine. They happen with various mag. nitudes at irregular time intervals. Over time, the cumulative effects of flooding gradually create a "floodplain'~ nat. ural overflow area adjoining each stream channel. Floodplains are as much a part of the hydrologic system as the channel iteself. Together, the channel and the floodplain make up the drainage path. way for each watershed (Fig. 4). Floodplains vary greatly in width, de. pending upon the size of the watershed and its runoff characteristics, the erosion potential of the land next to the stream channel, and the geologic age of the val. ley in which the stream is located. In narrow mountain valleys or canyons, floodplains may be small or virtually nonexistent where the stream flows be- tween rock walls. In more level or erod- ible terrain, streams characteristically layout broader floodplains~metirnes several miles wide in the case of major rivers. Figure 5. One floodplain Shared by Two States: The ConnectiCut River Serves as the Bounaary Between Vermont and New Hampshire (Photo: Sandra Hauptman) 7