GEOMORPHIC RESPONSE OF THE UPPER GILA RIVER, ARIZONA AND NEW MEXICO TO LEVEES, DIVERSION DAMS, AND FLOODS

Over the past century, the majority of alluvial reaches along the upper Gila River in Arizona and New Mexico have been leveed in an attempt to protect adjacent property from flood damage. In addition, the demand for irrigation has prompted the construction of diversion dams in these alluvial reaches to divert water for agriculture. Detailed investigation of historical channel change along the upper Gila River reveals that many of these channel modifications are catalysts for major channel change and can result in catastrophic property loss rather than safeguarding valuable farmland. In fact, there is a clear relationship along the upper Gila River between the scale of channel modification and property loss.

To quantify channel changes, channel widths were measured every kilometer for approximately 160 km from Safford Valley, Arizona through Cliff-Gila Valley, New Mexico for eight decades. An overall pattern of channel narrowing and widening coincides with periods of few large floods and periods of multiple large floods, respectively. In addition, reaches along the upper Gila River with greater channel modifications have experienced more variation in channel width than reaches with fewer modifications. Many channel changes in recent decades are unprecedented in previous historical aerial photography. These changes are consistently associated with artificial channel constrictions, such as levees, bank protection, and bridges, that have been built and rebuilt following large floods and that have accelerated natural channel narrowing during periods of few large floods.

Examples of geomorphic responses due to channel modifications along the upper Gila River include lateral erosion upstream of levees and diversion dams, redirection of flow over diversion dams into opposite banks, breaching of levees during floods and resultant erosion behind levees, channel widening downstream of levees, aggradation in leveed reaches, and lateral migration associated with straightened tributary channels.