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The relation of stream power and median grain size to the type of regime flow (modified from Benson and Dalrymple, 1967) is shown in figure 5. If the value 62RS V plots above the upper line, it may be assumed the upper-regime w flow occurs. If the stream power plots below the upper-regime line, a reliable n value cannot be assigned. Methods for evaluating depth-discharge relations for the lower- and transition-regime flows, which are beyond the scope of this report, are discussed in Simons and Senturk (1977). Stable Channels I , !. The base n values for a stable channel normally range from about 0.025 for firm earth to about 0.070 or larger for large-boulder channels and shallow depths as shown in table 2. These values are based on verification studies; however, the values have a wide range because the effects of bed roughness are difficult to separate from the effects of other roughness factors. The rough- ness of a reach in exposed bedrock can be evaluated in terms of the average height of rock protrusions above the bedrock surface and by using this value as an indicator of median diameter in table 2. Large boulders scattered in a stream need to be considered as channel obstructions. Personal experience and judgment will influence the selection of the base n value. Analysis of available verified stream n value data indicates n values vary with depth of flow (see section "Depth of Flow"). This is true for many streams in Colorado. Prediction equations have been developed to assess the base channel roughness to assist in the determination of n values from onsite inspection or from photographs. Two equations are presented, one by Limerinos (1970) for generally lower-gradient channels and one developed in this study for higher-gradient channels. In this report, a higher-gradient stream is defined as one having slopes greater than 0.002. The Limerinos equation is, however, valid on streams steeper than 0.002, but its upper limit has not been ascertained. Equation for predicting roughness coefficients of lower-gradient natural channels Limerinos (1970) related n to hydraulic radius and particle size, based on samples from 11 predominantly lower-gradient stream channels having bed material ranging from small gravel to medium-size boulders. Particles have three different diameters or dimensions--Iength, width, and thickness--and generally are oriented so that length and width are about parallel to the plane of the streambed. Limerinos (1970) related n to minimum diameter (thickness) and to intermediate diameter (width); his equation using inter- mediate diameter seems to be the most useful because this dimension is the one most easily measured by screening, by photographs, and by onsite evaluation. 21