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9 stretch. Instead of limestone, however, the rock is Weber Sandstone, which is easily eroded. Water diverted around the cobble bar flows with against this outcrop and has formed on overhang of about 25 feet. On the underside of the ledge moss grows, and is fairly thick and lush near the water surface at the upstream end. Most of the river flow passes through the left channel thalweg. At high flow the cobble island is completely submerged. Cross-section 5 is a deep pool with a sand substrate. The riffle initiates at cross- section 4 and extends to cross-section 2. Cobble substrate is found in both channels in the riffle with large cobbles and small boulders comprising the bed material in the left channel near cross-section 3. Both the cobble bars at river mile 16.5 and 18.5 are constructed of cobbles deposited in a flow expansion. The cobble bar at river mile 18.5 has considerably more sand covering the cobbles when exposed at low flow than at river mile 16.5. Mathers Hole, River Mile 17.5 Mathers Hole is located between the two previously described sites. It was established in a mild sloped reach of the river, which is partially armored with cobble substrate. Cobble bars or islands are located immediately upstream and downstream, dividing the flow and creating riffles where it is constricted between the islands and bedrock walls. This recurring riffle-pool sequence constitutes a reach of the Yampa River from river mile (RM) 16.5 to 20„5. Cobbles bars or islands have developed just upstream of each incised meander bend where the channel widens. The river is described as a wide bend, point bar stream with a distinct riffle-pool sequence. The site is reasonably straight with a short longitudinal cobble bar that becomes exposed at low flows. The left bank is a vertical Weber Sandstone wall, six hundred feet high. The right bank is a vegetated terrace rising approximately twelve feet above the cobble substrate. The river has entrenched a channel below this bench which is very stable up to bankfull discharge. The thalweg is permanently located near the left bank. During peak flows the water surface slope is uniform for several river widths downstream. A riffle-pool sequence develops with dropping stage and the water surface slope through the site is reduced. DATA COLLECTION Field Techniques Sediment discharge measurements were collected at Mathers Hole. These measurements constitute the sediment supply to the cobble substrate reach at river mile 16.5. Hydraulic measurements were made at cross sections established at river mile 16.5 and 18.5. Twenty-one additional cross sections were established at various sites in the canyon. These cross sections were monitored early in the spring during the rising limb and again, late in the fall as the discharge approached baseflow.