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e A ? 3 volcanic ash were accumulating to a thickness of over 7,000 feet. Between the Uinta and Yampa Faults, the eastern part of the Uinta Mountains was collapsing. The Browns Park Formation overtopped the valley rim, and the Green River breached the Uinta crest at the present site of Lodore Canyon. The river cut quickly through the soft Browns Park formation to harder rocks below. The main Green River was still flowing eastward across the rising Continental Divide. The Lower Green incised itself to its new base level, and eventually captured the main Green and turned it south. The Yampa River appears to have a similar history of incision into the Browns Park formation. The Yampa canyon profile varies distinctly between reaches with different rock lithologies. Two formations are seen most often in the canyon, the Morgan and the Weber. The Morgan consists mainly of limestone beds and underlies the Weber. The Weber is a relatively soft sandstone. Where the Yampa flows through the Morgan, the valley profile tends to be asymmetrical. On the south side, steep walls are found while the north side slopes gently and is covered with talus. The asymmetry is caused by lithology and the dip of the beds which is 7 to 10 degrees to the southwest. The lower member of the Morgan is an incompetent shale, which rests on the Round Valley Formation, a limestone. Where exposed by the river the shale slides on the Round Valley downslope, causing overlying rocks to collapse. Most landslides therefore occur on north slopes, in the Morgan. The river channel is confined between the steep talus slopes and has no floodplain. Channel location in the valley has been dictated in some reaches by the ancient landslides. In the Weber formation the canyon's profile is symmetrical. The Weber is a soft sandstone which is easily eroded by the river. Smooth, curving walls are often vertical or past vertical depending on the length of time they are subjected to the erosive forces of the river. Channel bed slope varies with rock type encountered. In the upper reach of the canyon, the slope is steep where the river flows through the Morgan. Boulders and talus from slides armor the river bed slowing its rate of downcutting through the hard limestone formation. The slope 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 downcutting in the Weber decreases as the channel elevation approaches the Green River base level (Figure 2). An average 1.5 million acre feet of water flow through the Monument each year in the Yampa, contributing on the average 1.5- to 2.0 million tons of sediment per year. Almost all of the sand sediment load is delivered by the Little Snake River entering the Yampa just four miles east of the Monument Boundary. This sediment load is transported through the cobble substrate reaches in the lower half of the canyon. 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.