FLUVIAL GEOMORPHOLOGY OF THE UPPER GILA RIVER AND ITS APPLICATION TO RIVER MANAGEMENT

Extreme floods along the upper Gila River during recent decades caused extensive erosion to agricultural property and damage to infrastructure in Duncan and Safford valleys. An interdisciplinary study employing geomorphology, engineering, and hydrology, was initiated to assess the cause of property damage and the stability of the upper Gila River. The geomorphic study of historical changes and surficial mapping addresses short-term and long-term stability in the upper Gila River system. Historical channel changes demonstrate that channel widths in recent decades are not unprecedented in the historical record. In fact, channel widths measured in 2000 are very similar to those measured from 1935 aerial photographs. The alluvial chronology developed along the upper Gila River spans about the last 3,000 years. The Gila alluvium is the youngest surface and is extensively inundated and modified during large floods. The Pima alluvium, a broad vertically accreted floodplain unit, is overtopped occasionally during the largest floods and is <1,000 years old. Alluvial surfaces that are older than the Pima alluvium are only marginally inundated by floods and thus define the Geomorphic Limit of floods. Alluvium that defines the Geomorphic Limit is about 1,000-3,000 years old.

Geomorphic data thus indicates that the upper Gila River has a well-defined area of lateral migration for about the last 3,000 years. Although most channel changes have reoccupied previous areas of the active channel or occur within the Gila alluvium, a few reaches with large-magnitude channel change are unprecedented in the historical record and have eroded major areas of Pima alluvium and minor amounts of alluvium along the Geomorphic Limit. The erosion in these reaches appears to be related to anthropogenic modifications including the construction of levees, dikes, bridges and diversion dams. The geomorphic study proved to be a powerful tool for understanding the upper Gila River. Based on this understanding, it is now possible to make sound river management decisions that minimize property loss and maintenance costs.