5.3.5 Snowmelt in mountainous areas

Snowmelt is discharged into drainages in mountainous terrain and into ponds, lakes, and relatively flat rivers in level terrain. Each drainage has a limit as to the amount of runoff it can handle, beyond which flooding will occur.

A mountain stream fed by melting snowpack.

The rate of melting depends on several factors:

  • The low-level temperatures of the atmosphere, with warmer temperatures leading to more melting.
  • The presence and amount of liquid precipitation. In general, rain melts snow when atmospheric temperatures are relatively warm. But when they are very close to freezing, the rain usually freezes as it percolates into the snowpack. The heat release associated with this freezing warms the pack to near the melting point so that any additional heating will melt it much more quickly.
  • The type of ground cover. If the surface is frozen soil or rock slab, little snowmelt will be absorbed; most of it will move horizontally, pool, or move downhill, increasing runoff. If the surface is unfrozen soil, the ground can absorb water more easily, decreasing runoff.

Of course, snowmelt runs off faster in sloped regions than flat areas.

Here Ethan Greene discusses springtime snowmelt: