intentional, such as the construction and maintenance of a large dam to control the return interval of large floods. More often, however, modifications occur as unplanned and unanticipated outcomes of watershed activities. Urbanization of watersheds ranging up to about 10 square kilometers (4 square miles), with attendant installation of impervious surfaces, streets, and drains, causes floods of a given magnitude to become more frequent than under previous natural conditions. During construction in urban watersheds, previously frequent sediment discharges of small amounts temporarily become much larger before declining in magnitude again after construction. In every case, small watersheds react to both natural and human changes more rapidly than large watersheds. Thus management plans for medium to large watersheds may not have immediately visible effects. Over periods of several years or decades, however, the fruits of wise watershed management become more apparent.

Hydrologic Units

Organization of policy-making bodies along the geographic lines of watersheds is not as difficult in its conception as it is likely to be in implementation. The boundaries of the nation's drainage areas have been precisely delineated and are available in printed and digital form. During the 1970s, the U.S. Water Resources Council devised a framework for dividing the nation into water resources regions wherein all the regional boundaries are hydrologic and topographic except where blocked by international boundaries (U.S. Water Resources Council, 1978). The regions contain either the drainage area of a major river, such as the Missouri Region, or the combined drainage areas of a series of closely related rivers and their watersheds, such as the South Atlantic-Gulf Region which includes a number of watersheds draining directly into the Atlantic Ocean or the Gulf of Mexico (Figure 2.6). Note that from a hydrological view, these water resource regions do not stop at international borders, as the map implies, which complicates both policy-making and implementation efforts.

The Water Resources Council's second level of classification divides the regions into 222 planning subregions. A planning subregion includes that area drained by a river system, a reach of a river and the tributaries in that reach, a closed basin, or a group of streams forming a coastal zone. All the planning subregion boundaries are hydrologic except where discontinued at international boundaries. In 1974 the U.S. Geological Survey, in cooperation with the U.S. Water Resources Council, published maps for each state showing the locations of the boundaries, as well as a national map. A few years later during the Second National Water Assessment, the Council reclassified the 222 planning regions into 106 assessment subregions, which have hydrologic boundaries that can be approximated by county boundaries. This connection between the physical landscape and the political landscape is critical, because it eases the aggregation and analysis of both environmental and social and economic data. The primary

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