Snow that is packed down over many years at high elevations becomes glacial ice, which slowly proceeds downslope like a frozen river, under the pull of gravity, and eventually melts to become part of streamflow at lower elevations. Glaciers exert a direct influence on the hydrologic cycle by slowing the passage of water through the cycle. Like lakes and groundwater reservoirs, glaciers are excellent natural storehouses, releasing water when it is needed most. Glaciers, however, can release water when you need it least. Glacier-fed rivers reach their peak during hot summer weather.
Snow exerts a marked effect on the distribution of streamflow throughout the year.
Instead of immediately infiltrating the soil or running off into stream channels as rainfall does, this water is first stored for several months. The relatively quick thawing of snow in spring causes peak flows, sometimes resulting in floods. Some of the worst and most unpredictable flooding occurs when ice that has not yet melted is carried along in the swollen rivers until it jams, blocking the flow of water and creating a lake behind the jam with attendant flooding. When the ice jam breaks, a tremendous amount of water is suddenly released downstream, and more flooding may result. The St. Lawrence River, which drains the Great Lakes.