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I: , . . 464 Topographic Response of a Bar '1 based upon assumed equilibrium conditions. Even the analysis of river channel change commonly focuses on prior and future equilibrium conditions, while neglecting the actua.l course of channel adjustment. Disequilibrium rivers are common; indeed, they may be the norm rather than the exception. In many instances, human activities, e.g., agriculture, forest clearing, grazing, urbanization, and the development of water supplies, have caused large, long-term changes in the quantity of flow and sediment supplied to the channel network. Natural events, also, can have a very significant impact on river equilibrium. Moreover, adjustment of river channels to such long-term trends frequently occur over a period of decades to centuries. Disequilibrium rivers are not only common, they are persistent on a human time scale. As our understanding of long-term trends in streamflow and sediment transport increases, assumptions of equilibrium are becoming progressively less justifiable. Realistic anl'lJysis of long-term channel changes requires theoretical approaches which are not based upon the existence of equilibrium conditions. In recent years, physically-based numerical models which accurately describe the flow structure and distribution of boundary shear stress throughout a nonuniform channel with arbitrary topography and longitudinally varying radius of curvature have been developed [see Smith and McLean, 1984; and Nelson and Smith, 1988a,bl. The divergence of the local bed;naterial transport field, which represents the ev01ution of channel topography per unit time, can be computed from the distribution of boundary shear stress. . NeLson and Smith [1988b] describe the simulated development of steady-statepoint-bars and alternating bars in an initially flat-bedded channel. This analysis does not require 'one to assume that equilibrium exists, and indicates the way to realistically analyze long-term channel adjustment. This chapter will describe the application of a fully nonlinear numerical model to analyze the evolution of channel topography in a reach of the Green River near Ouray, Utah, over relatively short periods of time at various discharges. The reach has a gentle, but complicated curvature and highly nonuniform bankfull channel width and area. The reach contains a prominent mid-channel bar which was the primary focus of our investigation. The bar is composed of well-sorted sand and is not stabilized by vegetation or debris. Green River The Green River drains approximately 116,000 square kilometers along the west flank of the Rocky Mountains in Wyoming, Colorado, and Utah (Figure 1). It is one of the principal tributaries in the Colorado River Basin. The main stem Green River originates in the Wind River Mountains of Wyoming, and flows southerly to its confluence with Colorado River near Moab, Utah. The principal tributaries of the Green River are the Blacks Fork, Yampa, Duchesne, White, Price, and San Rafael Rivers. Flow in the Green River has been rep;uJated by Flaming Gorge Reservoir since October 1962. Tributaries to the Green River have numerOUS small- I~"L - [impoundments, especially in their headwaters. Except for. the Duchesne -River, however, the tributaries are generally free flowing and unregulated at present (1988). _ { The location of the long-term gaging stations in the Green River basin are ~ '}fti'Jf \ shown in Figure 1. Water discharge has been recorded daily at most gages for several decades. Extensive records of suspended-sediment concentration also have been collected at the gages shown in Figure 1. The available historic information concerning flow and sediment transport in Green River provides one of the longest and most comprehensive descriptions for any river in the world. Andrews [19861 analyzed the water and sediment discharge records and determined mean annual