Irrigation diversions of the Verde River, Arizona: An assessment of impacts on water quality and riparian vegetation

The Verde River is one of the last free-flowing rivers in Arizona. In river ecosystems, discharge is the primary driver influencing environmental variables such as water quality, temperature, ecosystem connectivity, and geomorphology. Seasonal low flows and desiccation can be normal events for streams in arid and semi-arid regions such as Arizona. However, river diversions for irrigation can exacerbate the magnitude, duration and spatial extent of low flow conditions. Much research has been performed on the effects of large dams on river ecosystems, but there is little understanding of when or to what extent water diversion for irrigation alters the water quality and affects the ecosystems of Southwestern Rivers and the surrounding areas. Although, several options have been proposed for improving the water efficiency of river diversion, including lining the ditches, their potential impacts are not well understood.;The objectives of this study are (1) to quantify the difference in water quality parameters including conductivity, major cations and anions, nutrients, E. coli bacteria, and algal biomass in the Verde River above and below the irrigation diversions, and at the return flow in the diversion ditches; (2) to conduct a preliminary study on the changes in riparian vegetation abundance, and dominant land cover with proximity to the ditches; and (3) to make management recommendations for river diversion practices that maintain sustainable stream characteristics and stream ecosystems.;Research results indicate that diversion ditches do not contribute to changes in water quality of the Verde River. However, research documents that changes in water quality do occur from upstream to downstream along the Verde River with the inflow from Beaver Creek playing the major role. Likewise, this study clearly illustrates seasonal changes in water quality along the Verde River with a July sample having the highest concentration of major cations and anions. As predicted, this study found a difference in the abundance of mesquite and riparian trees at different distances from the ditches. A higher abundance of riparian trees was found 3.1 meters (10 feet) away from the ditches and a higher abundance of mesquite vegetation was found 15.2 meters (50 feet) away from the ditch compared to other plots.;It was obvious from this research that the concentration of major ions was overall higher for July when the discharge was lowest and irrigation demand was at its peak. With climate change predicted to bring more severe droughts in the arid region of Arizona, there is the potential to witness much higher change in concentration of these major ions with similar diversions in future. Thus, essential water management plans need to be implemented with future scenario, as drought stress is not only related to water scarcity but also reduces water quality. This research also documents the positive correlation between irrigation ditches and important native mesquite and riparian trees along ditch banks. More study should be conducted to understand the multiple functions and services these ditches provide before implementing major changes including lining the ditches. This study could inform water management plans and policies for semi-arid regions including Arizona.