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The selected n values need to apply not only to the cross section they are assigned to, but also need to be representative of the study reach under consideration. Consequently, the selection of cross-section locations and reach lengths needs to be carefully considered. Base Value The base roughness value of a streambed depends on the size, shape, spac- ing, and spacing pattern of the bed-material particles, channel gradient in natural stable channels, depth of flow, suspended material, and the bed level. Tables of base n values have been developed independently by several sources (Chow, 1959; Benson and Dalrymple, 1967; Aldridge and Garrett, 1973), but they are for different settings, such as stable, movable bed, and urban channels. These tables of n values will be presented in later sections. Particle Size Generally, roughness increases with increasing particle size; sand-bed channels have the smallest roughness values. Shape, spacing, and spacing pattern of the larger particle sizes are extremely difficult to quantify in terms of their effects on roughness and, therefore, are not considered directly in the evaluation of roughness. The size and size distribution of the bed material can be determined by either of the following methods, as described in detail by Benson and Dal- rymple (1967). If the bed material is composed of material of about 2 in. or smaller in diameter, small samples of the bed material needs to be collected at representative sites throughout the stream reach. A sieve analysis is made on the composite sample, the volume of material in each sieve size range is converted to a percentage of the total, and a size-distribution curve of the composite sample is prepared. If the material is too large for a sieve analysis, a grid system of 50 to 100 points per stream reach is used (Wolman, 1954). The width or intermediate diameter of a particle at each grid is measured and recorded. The sizes are grouped into a minimum of five ranges, and the number of particles in each range is recorded and converted to a percentage of the total. In both methods, the size that corresponds to the 50th percentile or median diameter (dso) is obtained from a distribution curve derived by plotting particle size versus the percentage of sample smaller than the indicated size. It often is not possible to physically measure the bed material because of deep or fast-flowing water or time constraints. In these cases, the median particle size needs to be visually estimated and recorded during inspection of the stream reach. 13