Alluvial fans are ubiquitous geomorphological features that occur throughout the world, regardless of climate, at the front of mountains as the result of erosion and deposition. They are more prominent in semi- and arid climates simply because of the lack of vegetative cover that masks their fan shapes in more humid areas. From both engineering and geological viewpoints, alluvial fans present particular fluvial and sedimentation hazards in semi- and arid regions because episodic rainfall-runoff events can result in debris, mud, and fluvial flows through complex and, in some cases, migratory channel systems. Further, in semi- and arid climates alluvial fans often end in terminal or playa lakes. Given the uniform topography of playa lakes, these features often present ideal locations for facilities such as airports; however, regardless of the engineering advantages of the topography, the episodic and often long-term flooding of these lakes attracts migratory birds. The purpose of this volume is to summarize the current state-of-the-art, from the viewpoint of engineering, in the identification and mitigation of flood hazard on alluvial fans; and to accomplish this a fundamental understanding of geology is required.

Stream channels in arid regions are subject to a wide range of hydrologic, hydraulic, and sedimentary conditions. These channels often are dry or have little streamflow most of the time, and the few flows that do occur can cause substantial changes to the channel and flood plain. Because floods in arid regions are often flashy, and many gaging stations are in remote areas, hydrographers must rely on indirect measurements of streamflow. Channel change is important because one major assumption necessary for indirect measurements of discharge is that the channel conditions after the flood represent the conditions during the peak discharge. The U.S. Geological Survey, in cooperation with the Flood Control District of Maricopa County, is monitoring selected perennial and ephemeral streams within Maricopa County, Arizona, to track the amount and variability of channel change. This report contains basic data from surveys of monumented cross sections conducted from 1997 through 2002. The amount of change varied widely from channel to channel, and the largest geomorphic change occurred in conjunction with peak flows above the 10-year recurrence interval.