Lake Mead is a 66,000 ha-deep subtropical reservoir located on the Colorado River, Nevada-Arizona. This largest U.S. reservoir by volume is limnologically complex with four inflows, three basins, plus variable seasonal and annual operational patterns. This investigation is about the most downstream basin, Boulder Basin. Thirty-four measured parameters are used to describe limnological conditions of Boulder Basin, including Las Vegas Bay, from 1990-2004. Some assessments use data sets from May 2000 to April 2004; other assessments use more extensive data sets spanning up to 14 years of records. Selected parameters, trends, basic and reservoir-specific processes, and limnologically important relationships are illustrated and discussed. Boulder Basin is well stratified and ecologically complex due to the existence of two interflows. A super- to mesotrophic gradient exists from the nutrient rich inner basin, which has inflow from an urban tributary, to the middle and outer basins, which become progressively less productive. Secchi depth increases, chlorophyll (Chl a), plankton abundance, conductivity and total organic carbon (TOC) decrease going from the inner to outer basins. Lake Mead is strongly phosphorus (P)-limited and subject to a variety of algae blooms that depend on the presence and amounts of phosphorus. Due to ecological complexity, managing this resource is difficult and fraught with risk when conditions change. Along with the changing hydrologic conditions of the watershed that heavily influence Lake Mead, man-made products and toxin-producing algae are being introduced that need monitoring and management.
