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r Andrews and Nelson 471 vertical and horizontal advection by the mean flow field. Although the full problem is tractable, often the computations can be greatly simplified and treated in a less computationally intensive manner. Where the suspended sediment concentration profile responds to spatial variations in boundary shear stress over horizontal length scales that are short compared to the spatial scales of the bed topography, quasi-uniformity may be assumed. This assumption implies that the local concentration profile is in equilibrium with the local boundary stress. Under such conditions, the analysis of Rouse 119371 is correct at the lowest order. This approximation is similar to one used in the flow model described herein, where the spatial distribution of acceleration and deceleration was assumed to have a weak effect on the vertical structure of the primary flow. Assuming a quasi-uniform vertical structure for the sediment concentration field allows the application of a total sediment load equation, wherein both the bedload and suspended load fluxes are calculated using a single expression. One of the better known of these equations is the Engelund-Hansen [1967] equation, given by .08 T.5/2 q, ~ = (1) f where q, is the volumetric transport rate per unit width, p, and p are the densities of the sediment and fluid, respectively, d is the particle size in transport, T. is the dimensionless shear stress, and f is the friction factor (drag coefficient). Given the distribution of boundary shear stress and the particle size, one can calculate the sediment flux at each location on the bed using the Engeiund-Hansen equation. A correction for the effect of form drag on the skin friction boundary shear stress is an implicit part of this equation. As a result, this equation can only be expected to retain validity where bedforms are similar to those in the experiments used in calibrating the equation. In addition, this expression can be expected to yield reasonable results only when the ratio of skin friction boundary shear stress to the critical shear stress is large (large transport stage), because it contains no critical shear stress dependence. Furthermore, the expression is applicable only where the spatial scales over which bottom stress varies are long compared to the distance a particle is advected while it settles. The Engelund-Hansen equation was calibrated using transport stages (ratio of boundary to critical shear stress) and sediment sizes and size distributions similar to those found in the Green River, and it can be expected to give good predictions of local sediment fluxes in the case of interest. Given the direction and magnitude of boundary shear stress at a point on the bed, the local vector sediment flux was calculated using the Engelund-Hansen equation. Then, the streamwise and cross-stream components of the local vector sediment flux were determined. This approach neglects the relatively weak redistribution of suspended sediment by secondary currents and is justified because most of the suspended sediment is near the. bed,wheJ;"e the flow direction is approximately the same as tbe boundary shear stress. Because a vast majority of the sediment flux is suspended, no gravitational correction was included.. . Evolution of Channel Topography Channel configuration may change with time as a result of two distinctly different mechanisms. First, the channel may evolve due to erosion and deposition of sediment which makes up the channel bed and banks. The so-called "erosion equation" relates the flux of sediment to the rates of erosion and deposition of the bed as follows: ".-.-',.-. . ,1tlIr:......, "l~~