Lake Mead, located on the border between Nevada and Arizona, is one of the largest constructed reservoirs in the world. Created by the Hoover Dam in the 1930s, it is the largest U.S. reservoir by volume at its full capacity of over 28 million acre-feet. This reservoir is a fundamental resource in the arid Southwest, supplying water to approximately 25 million people across multiple states and Mexico. The vast majority of its inflow is derived from a single source.
The Primary Source: The Colorado River
The Colorado River is the primary source of water for Lake Mead, accounting for approximately 97% of the reservoir’s total inflow. The construction of the Hoover Dam created the reservoir, transforming a long stretch of the river into a controlled lake.
The river’s flow enters the reservoir from the east, traversing basins like the Gregg and Virgin Basins before reaching the Hoover Dam. This constant, regulated flow acts as the primary conduit, sustaining the reservoir’s capacity. The river’s health and flow directly determine the water availability for the entire lower basin of the Colorado River system.
Fluctuations in the river’s contribution immediately and significantly impact the overall level of Lake Mead. The damming process harnessed the river’s power to create a reliable storage system, dwarfing all other localized sources combined.
The Mechanism of Supply: The Upper Basin
The water carried by the Colorado River to Lake Mead originates hundreds of miles upstream in the Rocky Mountains. This supply is rooted in the Upper Colorado River Basin, which includes parts of Wyoming, Colorado, Utah, and New Mexico. The primary driver of this water supply is the annual accumulation of mountain snowpack.
During the winter months, snow collects at high elevations, storing water in a frozen state. The subsequent spring and early summer warming trigger the snowmelt cycle, which releases this accumulated water into streams and tributaries that feed the main stem of the Colorado River. This seasonal runoff generates the majority of the flow that ultimately reaches Lake Mead.
Before this water reaches the reservoir, its delivery is heavily regulated by a series of upstream storage facilities. Most notably, the Glen Canyon Dam forms Lake Powell. Water is released from Lake Powell into the Colorado River, traveling through the Grand Canyon before arriving at Lake Mead. This upstream regulation ensures that the flow into Lake Mead is a managed release rather than solely a direct, uncontrolled pulse of snowmelt runoff.
Local Tributaries and Desert Runoff
While the Colorado River provides the bulk of the inflow, local tributaries and desert runoff contribute the minor remaining percentage of Lake Mead’s water.
Perennial Tributaries
The most significant secondary sources are the Virgin River and the Muddy River, which flow into the northern arm of the reservoir. Both are perennial streams, meaning they flow year-round, but their combined volume is small compared to the Colorado River. The Virgin River and the Muddy River converge before entering the Overton Arm of Lake Mead. Studies have indicated that the Virgin River contributes about 0.8% and the Muddy River about 0.1% of the total inflow. These flows are geographically important, influencing the water quality and ecology of the northern reaches of the lake.
Urban and Desert Runoff
A third localized inflow comes from the Las Vegas Wash, which enters Lake Mead at Las Vegas Bay. This wash carries urban stormwater runoff and treated wastewater effluent from the Las Vegas metropolitan area. The flow from the wash is highly variable and has increased over time due to the rapid population growth in the region. The surrounding Mojave Desert landscape also contributes highly variable desert runoff and flash flooding events. These events provide intense but short-lived spikes in inflow during periods of heavy, localized precipitation.