Scientists and resource managers are interested in water temperature, nutrient concentration, and other water-quality characteristics because they influence a range of ecosystem components, from support of aquatic bacteria and invertebrates to the behavior of fish. In particular, declines of Colorado River Basin native fish and changes in their condition have been attributed, in part, to low water temperatures downstream from dams, such as Glen Canyon Dam, that release water from deeper portions of the reservoir. USGS Grand Canyon Monitoring and Research Center and its cooperators monitor a range of water-quality parameters for Lake Powell, the Lees Ferry reach of the river below Glen Canyon Dam, and the mainstem of the Colorado River from Lees Ferry to Lake Mead.
The purpose of water quality monitoring in Lake Powell is to document and understand changes that occur while water is in the reservoir and how those changes may affect the quality of water released from Glen Canyon Dam. Water quality monitoring of Lake Powell has two components: monthly surveys of the forebay, the pool of water in front of the dam, and quarterly reservoir-wide surveys. Scientists collect data for basic water quality parameters, including temperature, specific conductance, dissolved oxygen, pH, oxide-reduction potential, and turbidity. Chemical and biological sampling is performed in some instances.
Water quality monitoring activities in the Lees Ferry reach assess the initial quality of water leaving the reservoir and entering Grand Canyon National Park. The parameters measured are similar to those monitored for Lake Powell and are important for detecting change and understanding the relationship between the quality of the water leaving the reservoir and the downstream aquatic ecosystem.
Water quality monitoring downstream of Lees Ferry includes the measurement of water stage and discharge throughout the river ecosystem in addition to basic water quality parameters. Suspended sediment concentration and grain size are also measured in support of research related to stable flow testing, evaluation of alternative fluctuating flows, tests of high flows, and ongoing development and evaluation of numerical modeling.
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