Evaluating water temperature & stream flow in the lower Virgin River, near Mesquite, Nevada, 2019-2021

Authors: Katherine Earp

Year: 2022

Abstract

The lower Virgin River is a shallow river that flows from southwestern Utah to Lake Mead in Nevada and Arizona. The Virgin River hosts several native fish species, including two endangered fish, woundfin (Plagopterus argentissimus) and Virgin River chub (Gila robusta seminuda), all which have seen reductions in population sizes since 1970. Reduced river flow (especially during summer low flow conditions) often results in higher stream temperatures, which can increase mortality, reduce breeding, limit population connectivity, and favor non-native species. This study investigated changes in water temperature and river discharge to provide pertinent data that directly impacts native fish species. The three-year monitoring project collected continuous temperature and discrete discharge measurements at 15 sites from 2019-2021. Preliminary results indicate that maximum summer water temperatures at many sites reach 34-36 degrees Celsius, several degrees above the published thermal maximum of 31 degrees Celsius for the survival of the endangered fish. During the study period, temperature was independent of flow at baseflow conditions, but temperature increased with reduced flows.