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54 shear stresses large enough for incipient motion of the median cobble size particle are necessary to scour the sand level to greater depths. In the field, the sand level was approximately one-half to one median cobble diameter below the surface of the cobbles. Dynamic Processes of Sand Transport over Cobble Substrate The following discussion applies the results and observations from the physical model study to the field conditions. The effects of deposition of excessive amounts sands and fine material on the cobble substrate can be severe, limiting the aquatic insect population, reducing the capacity for spawning, and reducing the channel carrying capacity. The processes which insure a sand free cobble substrate are closely linked to a seasonal hydrograph shape and duration. The cobble bars have evolved upstream of wide bends to produce relatively short riffles of steep slope and high velocities that prohibit sand deposi- tion. The pools upstream of the riffles have a sand substrate whose bed elevation fluctuates to seek equilibrium levels. The pool bed will scour at peak flows and will aggrade at low flow. There is evidence to suggest that the sand will migrate from a riffle to a pool during the base flow period (Rosgen, 1982). On the rising limb of the hydrograph, sands are deposited in the interstices. These sands are interchanged between the bed and the suspended zone for discharges less than bankfull. Depending on the supply-capacity relationship, either deposition or scour could be occurring. Coarser sand sizes are transported as bedload in tortuous paths around the cobbles. When the cobbles move, the sand, of course, is washed from the interstices and may completely be removed from around the cobbles. At that point, sand will be released only when one of the armor particles is moved. Rearrangement of the cobbles will result in more stability of the armor layer. On the falling limb, the armor layer becomes a trap for sands until finally, the sand reservoir is again filled. Without cobble movement, sand will be scoured only to a depth of one-half to one median cobble diameter below the cobble bed surface. It was observed that at peak flows, portions of the cobble substrate were covered with sand to depths of one foot or more. These channel areas, outside the thalweg, are subjected to deposition as the sediment supply approaches transport capacity. Discharges of one-half the incipient motion of the armor layer will be capable of extracting sands and fines from the cobble substrate (Milhous, 1982). This corres- ponds to roughly the effective discharge for the Yampa River. Mathematical Model Stud A quasi-steady water and sediment routing mathematical model wac- developed to simulate sediment transport in the cobble reach of the Yampa River. The field investigation of hydraulics and sediment transport was designed for the purpose of mathematically modeling a portion of the river to predict the river's response to various scenarios of simulated water and sediment discharge. Steepness precludes the necessity of modeling sediment transport in the first twenty miles of the canyon. A portion of the riffle-pool, cobble bar reach from river mile 16.5 to 20.5 was delineated for modeling.