Paleohydrology of pool and riffle pattern development : Boulder Creek, Utah

Abstract

The low-flow channel morphology at Boulder Creek is characterized by a well-developed pool-and-riffle pattern. The riffles consist of accumulations of basaltic boulders deposited from upstream source areas during extremely large flows. Paleoflood water-surface profiles defined by highwater indicators such as slackwater sediments and silt lines indicate that discharges of up to 400 ± 50 cms have affected the lower reaches of this stream system. Stratigraphic relationships and archaeologic and radiometric age constraints indicate that at least four large-magnitude, low-frequency flow events have occurred within the last 500 to 1000 radiocarbon years B.P. Step-backwater hydraulic reconstructions of these large flows suggest that the positions of the boulder-comprised riffles are controlled by spatial variations in large-flow stream power. Boulder deposition occurs where channel stream power drops below thresholds necessary for boulder transport. Reaches immediately upstream of canyun bends and constrictions, and downstream of canyon expansions are sites of large-flow stream-power minima. It is in the vicinity of these types of canyon geometries that the low-flow riffles are observed. Comparison of calculated stream power values and measured boulder sizes with established coarse-particle transport relationships indicates that a 400 ans flaw is approximately the minimum discharge that has the competence to affect this pool-and-riffle pattern.