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Channel Variations, nz Although the shape of a channel has little effect on roughness, changes in the size of cross sections and side-to-side shifting of the low-water channel in successive cross sections (also called channel alignment) will increase hydraulic losses. Gradual changes in channel dimensions do not increase turbulence; however, abrupt variations along the channel increase turbulence and need to be evaluated. Care needs to be taken not to reevaluate energy losses due to channel variations when making hydraulic computations with available computer models. Several models enable the user to calculate energy losses due to channel vari- ations as a function of contraction or expansion coefficients and the varia- tion of velocity head between successive cross sections. Obstructions, n3 Obstructions such as trees, stumps, large boulders, and debris deposits increase roughness and cause backwater upstream and eddy losses downstream. The degree of increased roughness can be evaluated in terms of reduction in cross-sectional area, which depends on the type, size, shape, number, and distribution of the obstructions. The effect of the obstruction increases with velocity as the area of the disturbance surrounding the obstruction increases and may overlap with nearby obstruction disturbances. Chow (1959) did not define the adjustments; therefore, adjustments for the four degrees of obstruction given in table 1 are based on guidelines provided by Aldridge and Garrett (1973). Free fall may result from the combined effect of several severe obstruc- tions that act like a weir; under these conditions Manning's equation is invalid. This problem can be avoided by the proper location of cross sections. Channel Vegetation, n4 The effect of bank vegetation is to increase turbulence and roughness and reduce channel capacity. At three of the sites (fig. 11), as the depth of flow in the main channel increased and encompassed bank vegetation, channel roughness increased. This is particularly true for narrow channels. The magnitude of this effect depends on the vegetation height related to depth of flow, the capacity of the type of vegetation to resist bending, the amount of vegetation that reduces channel capacity, and the time of year. Generally, the effect of the vegetal cover on resistance is greater during the growing season, which corresponds to the flood season in Colorado. During flood flows, floating debris commonly lodges in the vegetation and increases roughness. The criteria given in table 1 are based on the assumption that vegetation is uniformly distributed in the channel. If the vegetation grows in bands or is prevalent throughout a subarea in the reach, a composite n value needs to be assigned to each subarea and weighted according to the size of the wetted perimeter (eq. 3). 18