| Summary: | Changes in channel morphology of the ephemeral streams of the Rillito Creek system through time have constituted greater hazards for the Tucson metropolitan area than has overbank flooding. The behavior of this stream system has been investigated to document past channel instability, determine potential sites of bank erosion, and suggest floodplain management alternatives to the federal regulations currently applied to semiarid regions. Rillito Creek and its main tributaries, Pantano Wash and Tanque Verde Creek, were mapped from Houghton Road to the mouth of Rillito Creek using aerial photographs taken between 1941 and 1979. These maps, in conjunction with streamflow and channel composition data, detail channel change within this stream system. Prior to and including 1941, the Rillito Creek system exhibited braided plan-view patterns. By the early 1960's, single channel patterns had developed, accompanied by extensive decreases in channel widths. Pantano Wash continued to narrow to 1979 as its depth increased. Tanque Verde Creek and Rillito Creek widened extensively in 1965 and 1978 during prolonged winter flows, and narrowed during intervening periods dominated by low magnitude summer flows. Channel banks were eroded locally throughout this system during the periods of channel narrowing, however. The greatest amounts of bank erosion have occurred on the outer banks of channel bends and at locations where the silt-clay content of the banks and the density of riparian vegetation have been the least. In compliance with federal regulations, management of the floodplains of the Rillito Creek system has been focused mainly on those areas subject to flooding by 100-year floods. Frequent changes in channel morphology have created problems for floodplain management by altering the areas subject to flooding. In addition, bank erosion during lower magnitude flows historically has posed a greater hazard than has overbank flow. For more effective floodplain management, zones of potential bank erosion based on past erosional sites, historical channel positions, and current stream channel patterns should be considered for land-use regulation as well as potential areas of flooding. The principles established herein are applicable to other ephemeral stream systems in semiarid regions.
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