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i I INTRODUCTION General A morphological and hydraulic investigation of the cobble substrate reach of the Yampa River was conducted to determine the natural conditions and processes of the observed spawning habitat of the Colorado squawfish (Ptychocheilus lucius). Recent studies, conducted by the U.S. Fish and Wildlife Service (USFWS) and the Colorado Division of Wildlife (CDOW) have concluded that the cobble substrate reach (River Mile 10 to 19) of the Yampa River in Dinosaur National Monument is utilized as spawning habitat for the endangered Colorado squawfish (Tyus et al., 1982, Wich et al., 1983). The morphological investigation of the cobble reach was accomplished concommitantly with the National Park Service (NPS) study, "Hydraulic and Sediment Transport Investigation of the Yampa River." The study team also coordinated efforts to collect field data with the USFWS during 1983 runoff season which was submitted to the USFWS for input to physical habitat simulation computer model (PHABSIM). Results of this study and pertinent information from the NPS study are documented in this report. In the NPS investigation, a computer model was developed to simulate hydraulic and sediment transport conditions for a range of discharges in a portion of the cobble substrate reach. A minimum streamflow hydrograph was designed to preserve the existing cobble substrate conditions. A discussion of that hydrograph and its physical implications is presented herein. Setting _ Dinosaur National Monument is in the northwestern and northeastern corners of Colorado and Utah, respectively. It lies between Craig, Colorado and Vernal, Utah. The monument is about 70 miles long (east to west), and ranges from 10 to 25 miles wide (north to south). The Yampa River headwaters in the White National Forest on the western slope of the Rocky Mountains. It flows north, then west joining the Green River in Dinosaur National Monument (see Figure 1). Draining approximately 7600 square miles before entering the Monument, the Yampa is the Green River's largest tributary. The Yampa River is also the last major free flowing tributary of the Colorado River system. Background Dominating the landscape of Dinosaur National Monument are the Green and Yampa River canyons. Both rivers have been entrenching into bedrock since late Cretaceous time, about 70 million years (m.y.) ago. Prior to that time they were meandering rivers flowing on a mature plateau. The rivers still meander, but have incised within steep-walled canyons. The mechanics of meander entrenchment are not completely understood, but W. R. Hansen has a good theory for incisement of the Green River (1969, and personal communication, 10/7/83). He postulates that the Lower Green originally flowed from Browns Park into the North Platte River on the then-developing Browns Park Formation. Sediments and