Debris-fan formation and rapid modification at Warm Springs rapid, Yampa River, Colorado

In 1965 a debris flow from Warm Springs Draw, Colorado, deposited approximately 15,000 metric tons of sediment in the channel of the Yampa River, constricting the river against the opposite cliff and forming Warm Springs Rapid. Radiocarbon ages associated with several older debris-flow deposits bisected by the 1965 flow indicate a time interval of approximately 1000 years between large, channel-constricting debris flows from Warm Springs Draw. The older debris-flow deposits probably constricted the Yampa River in a manner analogous to that of the 1965 flow. The configuration of these tributary debris-fan rapids, which occur along canyon rivers throughout the western U.S., is a function of the sediment characteristics of the debris flow and subsequent hydraulic conditions within the channel constriction. Erosion of the debris-flow deposits and transport of the boulders through the rapid during different discharges was estimated with step-backwater modeling. Shear stress and critical flow best indicate the hydraulic conditions needed for entrainment and transport of the large boulders in Warm Springs Rapid. Results show that the present configuration of Warm Springs Rapid was formed during several high flows since 1965, principally the high flow of 1200 m(3)/s in 1984. Model results also suggest that flow duration may be as important as magnitude in modifying debris-fan constrictions. After modeling discharge magnitudes obtained from stream-gage records and slackwater deposits, we suggest that the geomorphic response time of the Yampa River to a disturbance of the magnitude of the 1965 event is less than 1000 years.