Paleoflood Hydrology of the Dolores River, Colorado and Utah

Field evidence from three paleostage indicators suggests at least four extreme flood events have occurred on the Dolores River in the past 2,000-years. The landscape position and sediment texture from the paleostage indicators reveal peak discharges that exceed those of the gauged or historical record. Additionally, evidence from one of the paleostage indicator suggests a high-discharge, hyperconcentrated flow event that occurred even more recently and lies ~8m above the present river elevation. Two of the paleostage indicators are slack- water deposits; one is in a sheltered cave and another on the lee side of a large rock outcrop. The other paleoflood deposit is an overbank slack-water deposit. At each slack-water site, we collected organic materials and sediment samples for radiocarbon and optically stimulated luminescence (OSL) dating. Preliminary correlations exist between two paleoflood units based on sediment color, texture and stratigraphic position. This is the first paleoflood study of the Dolores River and it is one of the final remaining, major tributaries of the upper Colorado River Watershed to be studied from a paleoflood perspective. The Dolores River is of particular interest because slack-water deposits located on the Colorado River suggest the Dolores River is the primary source for an extreme discharge event that occurred <2,000 years ago. That flood had an estimated discharge of 8,500 cm3s-1 (more than two-times the magnitude of the largest historical flood of 1884). The aim of this study is to determine correlations between the extreme flood that occurred on the Colorado River with the paleohydrologic record of extreme events on the Dolores River.