BOX 3–1

Stream Order

Stream order is inversely proportional to stream number. There are far more first-order streams than other streams. In many watersheds the first-order streams encompass 60% to 80% of the total watershed area. This relationship between stream order and stream number has been used to estimate the acreage of potential riparian wetlands adjacent to water courses. Brinson (1993) made this estimation and concluded that even though high-order streams had larger wetlands, the number of small-order streams compensated for the small size of the adjacent riparian wetlands. Thus, the total area of wetlands along small streams is similar to that for wetlands along higher-order streams. The difference, however, is that the small wetlands along first-order streams are more subject to developmental stress, a topic discussed later. In a study of the distribution of wetlands in a Maryland coastal plain watershed, Haas (1999) found that most of the wetlands were divided among three main topographic positions: headwater (above the channel head), at tributary junctions, and along the main channel. The wetlands at these sites had different average areas; the headwater wetlands were the smallest, and size tended to increase with stream order.

the distribution of riparian wetlands in the watershed, and the relationship between wetland functions and watershed position. Tidal channels are similarly organized, although with different stream order/area relationships than inland streams (Myrick and Leopold 1963; Rinaldo et al. 1999). In both drainage networks there is a minimum area required to sustain a channel (Montgomery and Dietrich 1988; Rinaldo et al. 1999).

WETLAND FUNCTION AND POSITION IN THE WATERSHED

The hydrological organization of the landscape into watersheds provides a context within which to evaluate the position and possible functions of compensatory mitigation wetlands. Wetlands occur in a variety of physical settings, including coastal lowlands, topographic depressions, broad flats on interstream divides, the base of slopes, and topographic highs with little slope (Winter and Woo 1990). Location in the landscape influences geological characteristics, such as slope; thickness and permeability of soils; and the composition, stratigraphy, and hydraulic properties of the underlying strata, all of which influence surface and subsurface flows of water.

Degradation of wetlands contributes to an overall decrease in watershed ecological function. Watershed scale can include river basins, subbasins or smaller hydrological units or drainage areas, the size of



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement