Mountain snow fields act as natural reservoirs for many western United States water-supply systems, storing precipitation from the cool season, when most precipitation falls and forms snowpacks, until the warm season when most or all snowpacks melt and release water into rivers. As much as 75 percent of water supplies in the western states are derived from snowmelt.
During certain times of the year water from snowmelt can be responsible for almost all of the streamflow in a river. An example is the South Platte River in Colorado and Nebraska. Historically, the South Platte River was essentially "turned off" after the supply of water coming from melting snow was exhausted in late spring. Today, though, seepage of irrigation water from ditches and fields replenishes the alluvial aquifer (water-bearing deposit of sand and gravel left behind by a river) during spring and summer, and the aquifer slowly drains during fall and winter by discharging groundwater to the South Platte River. Indirectly, your buying a loaf of wheat bread in the grocery store helps to keep water flowing in the South Platte River all year long.
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