Laserfiche WebLink
32 <br />development described in Sect. 2.4.1. This method has been found to <br />be particularly desirable in modeling large, unwadable rivers without <br />historical flow records (White 1976). <br />2.6 Usable Width <br />The terms 'usable width' and 'weighted usable width' originated <br />from assessment methods developed for coldwater, salmonid streams in <br />Oregon (Thompson 1972, 1974; Sams and Pearson 1963). The usable width <br />method is oriented to single transects across critical locations in <br />the stream channel where limiting habitat (e.g., spawning beds) <br />occurs. Instream habitat condition is quantified as the percentage of <br />the total stream width which is "usable," as determined by target fish <br />species, life stage-specific criteria, and field survey data. <br />Habitat-discharge relationships are obtained by repeating transect <br />surveys at several discharges. Originally, the method did not include <br />simulation techniques. <br />This type of approach is used for assessing flow requirements for <br />fish passage, spawning, incubation, and rearing (i.e., growth and <br />feeding). Applications are limited primarily to salmonid species. <br />Habitat criteria consist of velocities and depths that are associated <br />with the presence of fish. The difference between the usable width <br />(UW) and weighted usable width (WUW) methods lies with the type of <br />criteria used to define habitat usability. The former employs a <br />binary criterion (usable or unusable); for example, portions of the <br />transect that are less than a minimum depth criterion and exceed some <br />maximum velocity criterion have no habitat value. Examples of these <br />criteria for several salmonid species are given in Table 2-2. <br />1