Monitoring of responses to a local base-level change in an ephemeral stream

Abstract

A long-term program has been established in the Tucson Mountains to monitor changes caused by removal of a small dam whose reservoir was filled with sediment, and to describe the characteristics of the associated fluvial system. For this ephemeral stream, data are collected using cross-channel profiles, scour chains, longitudinal profiles, plane-table mapping, painted rocks, crest-stage gages, and measurement of sediment-size distribution. The drainage basin, described using topographic maps, has an area of 3.78 km², and length of 3910 m, a total relief of 547 m, an elongation ratio of 0.56, and a drainage density of 10.4 km/km². Three small post base-level drop flow events resulted in net deposition (with widely interspersed short reaches of net erosion) and increases in width-depth ratio downstream from the base-level fall, and formation of a gully and headcuts, local steepening of the stream gradient by 145%, and a decrease in the width-depth ratio by roughly 80% upstream from the base-level fall. The morphologic changes at intervals along the length of the streambed in response to the 2.5 in high base-level fall. The data appear to indicate a greater change in gradient than in width-depth ratio during initial response to the base-level fall.