Lake Mead, Colorado River, Arizona-Nevada, is one of the most heavily used reservoirs in the Western United States, providing abundant recreational opportunities as well as downstream domestic and agricultural water for over 22 million users. Based on average nutrient levels and productivity, Lake Mead is classified as mildly mesotrophic. The insertion into the water column of the Colorado River dominates the limnology of much of the 106 km-long reservoir, and may still be identified at Hoover Dam under certain conditions. The lower basin of Lake Mead ending at Hoover Dam is known as Boulder Basin and is near the Las Vegas metropolitan area. Las Vegas Bay, which comprises the northwestern portion of Boulder Basin, receives all runoff from the Las Vegas Valley via Las Vegas Wash. Inflow includes urban storm flows, secondary and tertiary treated municipal sewage effluent and groundwater seepage. The rapid increase in population of Las Vegas Valley and subsequent increased volume of point and non-point sources to Las Vegas Wash have produced noticeable physical and ecological changes in Las Vegas Bay, including eutrophication. However, the influence of the intrusion from Las Vegas Wash, which is easily identified by its relatively high specific conductance and turbidity, extends from Las Vegas Bay out into Boulder Basin, and at times, to Hoover Dam. The intrusion's depth, thickness, and the distance to which it extends into the reservoir depend on the season of the year and are related to the degree of thermal stratification within the reservoir. Limnological data on the extent of the intrusion to station LV14 suggest future studies should focus on examining the potential threat, under certain conditions, to subsurface municipal water intakes located at Saddle Island 2.5 km south of sampling station LV14. Concentrations of bacteria and organic compounds are higher in the intrusion than surrounding deeper or shallower water.