Geomorphology and hydrology of the Santa Cruz River, Southeastern Arizona

Abstract

The Santa Cruz River in southeastern Arizona is typical of large rivers of the semiarid southwestern United States. The 390-kilometer long river displays great variability in morphology, hydraulics, hydrologic, conditions, and bank stability, reflecting incomplete integration of the drainage system and the disequilibrium that is often characteristic of desert channels. Spatial variability is paralleled by temporal variability at all scales, from instantaneous conditions to millenia of geologic history. The alluvial history of the Santa Cruz River drainage basin suggests that transverse sediment transport toward the valley across alluvial fans has dominated fluvial processes for most of the history of the basin. No evidence is apparent for external drainage of the basin before the beginning of Fort Lowell Formation deposition, about 2.0 to 2.5 million years ago. The mid to late Pleistocene history of the system is poorly understood because of the paucity of the geologic record for this period. Several episodes of terrace formation apparently represent climatically induced sediment pulses and subsequent incision. The Holocene record of the Santa Cruz River suggests a major change in hydrologic regimen about 5,000 years ago when deposits indicate a substantial decrease in stream power. The last 2,500 years are marked by a sequence of channel cut and fill episodes similar to the historic episode of channel entrenchment that began in the 19th century. The hydrologic regimen of the Santa Cruz River is dominated by the occurrence of floods in direct response to precipitation. Three major storm types—monsoonal, frontal, and dissipating tropical—account for most streamflow. The storms cause floods with distinctly different characteristics. Monsoonal storm-caused floods may have quite high peak discharges, but are local and of short duration. Frontal and tropical storms cause floods of longer duration and flood volume and are more efficient agents of sediment transport and geomorphic change than are those caused by monsoonal storms. Geologic controls are the major factor governing the spatial variability of channel morphology and change. Hydrologic factors control the temporal variability of channel processes. Frontal and tropical storms are more likely to produce floods capable of causing and maintaining a continuously entrenched main channel. Floods caused by monsoonal storms do not appear capable of maintaining such a system.