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A Study of Impact of Urbanization on Ephemeral Streams in Headwater Watersheds in Eastern Pima County, AZ.In this study, the impact of urbanization on ephemeral streams in arid environments was assessed by comparing eight small headwater watersheds with different degrees of urbanization in Tucson, Arizona. Chronological change in urbanization, stream network, and downstream channel width were observed, employing GIS, orthophotos over thirty years, and field measurements. A decrease in drainage density caused by urbanization and a decrease in channel width were found in many study sites. The relationship between the chronological change in downstream channel width and the urbanization pattern in the watersheds was assessed using an index named Stream Based Urbanization Index that indicates spatial distribution of urbanization in watersheds in relation to streams. Based on the comparison of seven Index factors, a lower rate of decrease in channel width was observed in the watersheds with a higher urbanization, especially with a higher concentration of urbanization near streams among the general trend of channel narrowing.