The Santa Cruz River, an ephemeral river that drains 8,581 square miles in southeastern Arizona, has a long history of channel instability. Since the late 19th century, lateral channel erosion has caused extensive property damage, particularly in Pima County. During the flood of 1983, about $100 million damage was caused in the Tucson area alone; most damage resulted from bank erosion on the Santa Cruz River and its tributaries. The nature, magni- tude, location, and frequency of channel change on the Santa Cruz River were highly variable in time and space from 1936 through 1986 along a 70-mile reach in Pima County, Arizona. Four mechanisms of lateral channel change--channel migration, avulsion and meander cutoff, channel widening, and arroyo widening--were identified on the Santa Cruz River. The dominant mechanism in a reach depends on channel morphology and flood magnitude. The dominant vertical change has been degradation. The timing and magnitude of channel change at a particular location are controlled primarily by hydroclimatic factors such as magnitude, duration, intensity, and frequency of precipitation and floods. The location of channel change and its magnitude in response to a given discharge are controlled largely by topographic, geologic, hydraulic, and artificial factors. Although much of the present morphology of the Santa Cruz River is the result of recent large floods, a direct link between hydroclimatic con- ditions and channel change is not always evident because resistance of the channel to erosion varies with time. (USGS)
