Preliminary Hydrologic Investigation of Topock Marsh, Arizona 1995-98.

Abstract

A preliminary hydrologic investigation of Topock Marsh, located in northwestern Arizona, was conducted for the U.S. Fish and Wildlife Service between 1995 and 1998. The study identified the hydrological components of the marsh system, quantified an annual water budget, defined the water quality conditions of the open-water marsh, examined water management concerns, and provided data for a concurrent submersed aquatic vegetation (SAV) study. The water budget tabulated monthly values for all hydrologic components except net ground-water flow, which was determined by difference. The principal components were river inflow (+60,020 ± 1,020 ac-f1), rainfall (+890 ± 40 ac-f1), evapotranspiration (-28,170 ± 940 ac-fl), storage (-980 ± 30) and marsh outflow (-26,860 ± 890 ac-f1). The net ground-water value (-6,960 ± 1,650 ac-f1) suggests that the marsh recharges ground-water. The Refuge also exceeded its diversion and consumptive use entitlement. Water quality conditions were assessed using continuous and discrete sampling methods. Three in-situ devices measured routine water quality parameters (temperature, pH, conductivity, dissolved oxygen, oxidation-reduction potential) for two years. Quarterly water samples (1997-98) from four marsh sites were analyzed for major and select minor ions, and δ¹⁸O- δD values. The continuously monitored parameters exhibited, to varying degrees, temporal (diurnal and seasonal) and spatial variability. The marsh was well mixed and has oxygen levels capable of supporting most aquatic life. Evaporative concentration was the major process controlling major ion chemistry. Isotope data confirmed that the marsh and nearby shallow ground-water derive from a river-water source. The marsh is normally turbid (30 NTU), apparently resulting from the resuspension of detrital material by wind-generated waves. The turbidity values dropped below 5 NTU during the 1996 summer as dense patches of SAV "filtered" the water. Light penetration is probably a limiting factor controlling the distribution of SAV in the marsh. Today's water management plan is the culmination of historical practices, incomplete science, institutional goals, and other constraints. Together these factors have led to relatively static hydrologic conditions in the marsh. Potentially negative consequences can result from the continuance of traditional management, including loss of diversity, accelerated in-filling, and salinization. The Refuge should consider simulating disturbances (e.g., flooding) and developing a more complex water management plan.