Lake Mead stores Colorado River water for delivery to farms, homes and businesses in southern Nevada, Arizona, southern California and northern Mexico. About 96 percent of the water in Lake Mead is from melted snow that fell in Colorado, Utah, New Mexico and Wyoming.

Each year, Lake Mead receives a minimum amount of Colorado River water from these states, known as the "Upper Basin" states. And each year, a specific amount of water is released from Lake Mead to users in Nevada, Arizona, California and Mexico. In an "average" year, the amount of water flowing out of Lake Mead exceeds the amount of water flowing into Lake Mead.

In 2000, for example, the runoff was only 56 percent of normal. The runoff has continued to be well below normal. Because of this decreased runoff, Lake Mead has received only slightly more than the minimum required amount of water from the Upper Basin. But the amount of water going out from Lake Mead has remained at normal levels. So, there has been more water going out of Lake Mead over the past decade starting in 1998 than there has been coming into the lake. This causes the elevation to drop a little more each year.

The variation in water flowing into Lake Mead and the water flowing out of Lake Mead causes the lake's water level or "elevation" to fluctuate yearly and over multi-year periods, and it has done so throughout the reservoir's 70 plus year history. This is normal, and it is how Lake Mead was designed to work.

Water is released from Lake Mead only to meet downstream municipal and agricultural demands. Consequently, power demands in California, Arizona and Nevada do not impact its elevation.

Lake Mead is typically at its highest yearly elevation in the late fall and early spring months. The lake begins to drop in elevation in the late spring and early summer when the desert heats up and causes a higher demand of agricultural and municipal water needed in the Las Vegas Valley, Arizona, California and in Mexico. Some years, the drop is greater than others, depending on how much difference there is between inflow and outflow.

If there are several consecutive years where outflow exceeds inflow, Lake Mead begins each year with lower water levels, and the elevation continues to drop until a "wet year" occurs in the Colorado River. Then, Lake Mead typically receives more water than it releases, and the lake again returns to higher elevations.

This pattern - where the lake periodically fills to capacity then experiences a period of declining levels, only to fill up again - is projected to continue into the future. But no one can predict the weather, so it is not possible to predict when the high and low periods will occur.

Information furnished by the Bureau of Reclamation

• NASA Satellite Images