Lake Mead National Recreation Area: Limnological and riparian resource condition assessment

Ryan, Ka; Wittmann, Marion; Chandra, Sudeep; Turner, Kent; Sappington, Mark; Thom, Theresa

Natural Resource Condition Assessments (NRCAs) strive to report current conditions of important natural resources and indicators for National Parks. The Lake Mead National Recreation Area (NRA) Limnological and Riparian Resource Condition Assessment (LRCA) is a collaborative effort between the National Park Service (NPS) Lake Mead NRA staff and the University of Nevada at Reno (UNR). The goal of this report was to synthesize and interpret existing scientific data to evaluate the current status and forecast future conditions for limnological and riparian resources of lakes Mead and Mohave. Lake Mead NRA was established in 1964 as the nation’s first National Recreation Area. The park is located within Clark County, Nevada and Mohave County, Arizona, with Grand Canyon National Park and Grand Canyon-Parashant National Monument adjacent to the east. Three rivers feed into Lake Mead: the Colorado River provides the greatest inflow from the Grand Canyon to the east, and the Virgin River combines with the Muddy River and flows into the Overton Arm of Lake Mead to the north. Lake Mead is the largest reservoir by volume in the United States and provides domestic water to 25 million people throughout Nevada, California, Arizona, as well as irrigating 2.5 million acres of cropland. Lake Mead NRA is in the top five highest visited units in the National Park System, providing significant economic value to the region, and boasts 185,000 acres of designated wilderness and backcountry. Lake Mead NRA, as part of an inter-agency partnership, has committed to collect monitoring data since 2010. The active assessment for this LRCA began in 2015. The Long-Term Limnological and Aquatic Resource Monitoring and Research Plan, or LTAP (2010) provides a basis for the resource measures in this assessment. The Environmental Protection Agency’s (EPAs) framework for Assessing and Reporting on Ecological Conditions aided in determining indicator measures for riparian and limnological health. This assessment examines six Essential Ecological Attributes (EEA) that incorporate ecological structure, composition, and function of ecological systems at a variety of temporal and spatial scales: • Lake and Riverine Ecosystem Health • Biotic Conditions • Ecological Processes • Recreation Suitability • Stressors and Disturbance Regimes For each of the five EEAs, contributing resource attributes within Lake Mead NRA were defined. This resulted in 22 key resource attributes, with 32 limnological and riparian indicators and 45 associated measures of condition. Conditions and trends of each overall EEA were assigned based on the assessments of respective resource attributes and indicators. At the EEA level, Lake and Riverine Ecosystem Health and Recreation Suitability were in overall good condition with declining trends. Uncertainty about maintaining Lake Mead-wide monitoring programs and future climate projections contributed to the downward trends and uncertainties in the assessments. Biotic Conditions and Stressors and Disturbance Regimes each warranted moderate concern. The introduction of invasive species and monitoring contaminants were the primary concerns leading to the moderate concern category for these resources. Ecological Processes warranted significant concern with a downward trend. This attribute condition is driven by the indicator hydrologic regime, which warrants significant concern due to persistent drought or drought-like conditions throughout the Colorado Basin. Further details on the condition and trend for each key resource under the EEAs are detailed in this report. Evaluations of data collection and monitoring efforts have also resulted in the recognition of emerging issues and data gaps. First, though there is much data on the hydrologic properties of Lake Mead NRA, there are many unknowns regarding the entire hydrologic system surrounding lakes Mead and Mojave. Management of Hoover dam, the Colorado River Basin, and conditions...

Lake Mead National Recreation Area: Limnological and riparian resource condition assessment

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