HISTORICAL GEOMORPHIC RESPONSE TO LARGE FLOODS AND ANTHROPOGENIC CHANGES, UPPER GILA RIVER, ARIZONA

In recent decades, large floods on the upper Gila River in east-central Arizona have altered channel morphology. In addition to large floods, levees, dams and other modifications to the natural channel have further changed river morphology. To quantify these changes, channel widths were measured every kilometer in the 100-kilometer alluvial reach through Safford Valley and Duncan Valley, Arizona using aerial photographs from 1935 through 2000. The purpose of these measurements was to document channel changes during the historical period and to investigate the causes for these changes.

From 1935 to the early 1960's, a period of few large floods, the channel narrowed by sedimentation, vegetation growth, levee and dike constrictions, and agricultural development. Average widths decreased from 670 meters to 406 meters in Safford Valley and from 219 to 150 meters in Duncan Valley. This was followed by a series of large floods from the late 1960's to 2000 that resulted in significant channel change. Geomorphic response to large floods includes overbank channel splays, channel widening, and lateral migration from tributary alluvial fan deposition. Geomorphic response to anthropogenic changes includes lateral erosion upstream of levees and diversion dams, redirection of flow over diversion dams into downstream opposite banks, lateral migration of channel meanders, and channel widening downstream of dikes and levees. Changes in channel width and position in recent decades have resulted from more frequent, high-magnitude floods and are not unprecedented in the historical record. In fact, channel widths following the floods of 1960-2000 are very similar to 1935 channel widths. The results imply that the upper Gila River readily adjusts to the largest floods. In periods of few large floods, the channel narrows. This variation in width occurs over a decadenal time scale. Anthropogenic changes contribute to channel narrowing during periods of few large floods. During periods of large floods, some anthropogenic changes may result in the redirection of the channel to locations not occupied historically.