Assessing Stream Health Indicators

Abstract: The Tuolumne River watershed, ranging from Yosemite National Park in the high Sierra Nevada mountains to the central valley floor, contains multiple dams that provide a range of services from water supply and hydroelectric generation to recreation. We studied the impacts of these dams on the Tuolumne river ecology by assessing a series of stream health indicators in regulated and unregulated portions of the watershed. Data was collected on channel bed substrate distributions, algal biomass, benthic macroinvertebrates, and fish assemblages as indicators of overall stream health. Our findings indicate that dams, and especially daily pulse flows released below the dams, alter the variables for which we collected data. We provide some information on the costs and benefits to environmental flows in the Tuolumne River and suggest management of water releases below dams can be shaped to sustain essential ecosystem functions, without severely altering the economic and water delivery benefits. Implementing changes to the flow regime that will benefit the ecosystem, such as better mimicking the characteristics of the spring snowmelt recession or limiting pulse flows to the appropriate high flow season, is attainable and affordable within the Tuolumne watershed.