The relation between ephemeral stream regime and earth fissuring in south-central Arizona

Abstract

Earth fissures in alluvial basins of south-central Arizona, formed in response to ground-water decline and land subsidence, act as water and sediment sinks for periods in excess of ten years. Investigation of the relation between ephemeral stream regime and earth fissuring revealed aspects of the subsurface dimensions of fissures as well as the potential hazards of fissuring. Fissure and gully erosion was measured and mapped. Up to several hundred cubic meters of eroded material per hundred meters of fissure has been absorbed by fissures. Measured rates of fissure-induced erosion were significant when compared to rates of arroyo erosion determined in previous works. Ephemeral stream discharge and sediment transport were gauged with simple and inexpensive methods. Summer thunderstorm runoff events lasting approximately 30 minutes were observed to have up to 0.5 cubic meter per second peak discharge and transport up to 4 cubic meters of sediment per single small channel. Measured and estimated volumes of sediment deposited in earth fissures indicated fissure depths exceed 100 meters, sediment spreads out laterally within fissures, and fissures are unlikely to be much less than 3 centimeters wide at depth. Beneficial ground-water recharge through earth fissures was not deemed to be significant. However, observations indicated that potential ground-water contamination through fissures may be the most serious hazard associated with fissuring.