Geography of Water in the West
 

The Southwest United States is the most arid part of the country.  The massive Rocky mountains and other ranges in Colorado and New Mexico rise out of the plains and form the peaks at the Continental Divide.  These mountains have some lush valleys that recede into the arid basins of Nevada, Utah and Southern California.  Water supply in these arid and semi-arid regions is limited and variable.  Most of the precipitation comes from Wyoming, Colorado, and northern Arizona and New Mexico.  These areas get over 20 inches of rain per year while other arid parts of the southwest receive from 7 to 11 inches per year.  Many areas of southern California, Utah and Wyoming receive 0 to 10 inches per year.  The rocky mountains of Colorado receive the most rain of all 7 basin states, averaging over 20 to 40 inches per year.

The rainfall in these areas varies widely from season to season and year to year.  Much of the rainfall received in central Arizona falls between July and September.  Most of this water is lost due to evapo-transpiration so surface runoff is limited.  In other areas of the Southwest variation in stream flows is due to the spring melt of snow pack in the mountains.  Besides rain and snow melt, another source of water in the Southwest is groundwater.  Aquifers are present in southern Arizona and in Texas.  These are not a renewable source of water and are rapidly being depleted.

The Geography of the Glen Canyon National Recreational Area is quite unique to it's surrounding enviornment.  Being a lake 186 miles (344 km) long, and having 1,960 miles (3,630 km) of shoreline, it has quite a presence in this arid region.  The lake and the cool water below the dam are geographic anomolies in the desert that support a unique ecosystem.

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