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5 is steeper than in the reach where the softer Weber Sandstone is the controlling formation. Here the river often directly attacks the bedrock, which it erodes easily. The rate of downcut.ting in the Weber decreases as the channel elevation approaches the Green River base level (Figure 2). Site Description River Mile 16.5 Three study sites were selected in the cobble bed reach of the Yampa River. River mile 16.5 and 18.5 were chosen for hydraulic investigation of the cobble substrate. Hydraulic and sediment transport data were collected at the Mathers Hole site located between the other study sites. A description of the sites follows. The cobble bar at river mile 16.5 is a large cobble and sand longitudinal bar (approx. 900 ft. long and 300 ft. wide). It splits the main channel into two smaller channels (or three at high flows-see Figure 3). The Weber Sandstone is exposed through the entire cobble bar reach, attaining a maximum thickness of about 700 feet on the left bank. Talus and soil is found at wall bases and supports grasses, junipers, and box elders. The Weber is poorly consolidated and thus easily eroded, forming steep, smooth, curving walls. On the left bank is a vegetated flood terrace which extends to the midpoint of the cobble island, where it meets Weber talus and some sand. Appearing below cross-section 4 (Figure 3), on the right bank, is a limestone bed in the. Morgan Formation (lower Pennsylvanian). It dips upstream at about 6°, rises through the remainder of the section, and is about 10 feet thick. The outcrop does not become prominent until about 80 feet above cross-section 2. This outcrop is far more erosion- resistant than the Weber Sandstone, and extends into the channel rather than being sharply cut off. Here it accumulates muds and supports algae. On the left bank only a small portion of the limestone is seen, just above cross-section 1, but it quickly disappears under talus. The bed material is essentially cobbles from cross-section 1 to 5, with exception of the large pool in the left channel near cross-section 2 which consists of sand and boulders and a large portion of the right channel which is small boulders from the talus slope. Cross-sections 6 through 8 constitute a pool reach with a large percent of the substrate being sand. The pool is at cross-section 7 is deep with large submerged boulders. The riffle extends from cross-section 5 to cross-section 1 and includes both channels. River Mile 18.5 At river mile 18.5 is another cobble, riffle reach which is similar to that at mile 16.5. A longitudinal bar of cobbles and sand splits the main channel into two subordinate channels (Figure 4). The Weber Sandstone is exposed through this reach with no trace of the Morgan Formation. As with the site at mile 16.5, an overhanging ledge is found at mile 18.5. It rises from the water about 50 feet below the uppermost point of the bar, and continues to rise throughout the remainder of the