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55 The sediment supply for the model was the 1983 measured sediment data at Mathers Hole. Two cross sections downstream were surveyed as intermediate cross sections to the modeled area. Eight cross sections were monitored at the cobble bar area at river mile 16.5. Five of the eight were surveyed over the cobble riffle and three through the pool upstream (see Figure 3). In all, approximately one and one eighth miles of the four mile cobble reach were modeled. The model reach contained 3 riffles and 3 pools. The last riffle-pool sequence was modeled in its entirety. This sequence represents the complete range of hydraulic and sediment transport conditions and processes that are found in the four mile reach. The model is constructed of a series of components, which have been universally applied in engineering predictions of sediment transport. It preserves continuity by keeping a budget of the incoming sediment supply, the transport capacity and sediment deposition or scour in each individual reach. The routing model is designed for determining channel aggradation and degradation in a river system. The river is divided into a series of computational reaches with similar hydraulic and geomorphic characteristics. Hydraulic conditions for each reach are calculated using the well-known and widely applied U.S. Army Corps of Engineers HEC-2 water surface profile computer program. The computed hydraulic conditions are then used to calculate sediment transport capacity for each subreach in the downstream direction. The sediment transport capacity is compared with the sediment supply from the previous subreach and the resulting aggradation or degradation is uniformly distributed both laterally and longitudinally in the reach. The input discharge hydrograph is discretized into a series of time steps; each step represents a period of steady discharge. The sediment supply to the first cross section is similarly discretized for the same time steps. A new value for water and sediment discharge can be inputed with each time step to simulate the gradually varied flow condition. A description of the sediment transport processes, model assumptions and model calibration are presented in Appendix B. The results of the mathematical model study was a minimum streamflow hydrograph based on the predicted aggradation/degradation response to reduced water discharge. The minimum streamflow hydrograph was contructed to insure that a relatively sand free cobble bed is maintained in the modeled reach during the period from mid-July through mid-August. This criteria should be met on an annual basis for any given sediment supply scenario. The sediment supply to the upstream cross section, represented by a sediment input hydrograph, governs the resultant shape and timing of the simulated minimum streamflow hydrograph. Numerous sediment supply scenarios and water discharge hydrographs were tested utilizing both sediment-water discharge regression relationships and sediment hydrographs. In the final analysis the simulated hydrograph was based on the 1983 measured sediment load at Mathers Hole.