Permafrost and Water Runoff

In temperate climates, precipitation tends to percolate into the ground rather than to run off in open stream flow. Less than 20 percent of precipitation falling on the watershed of the Mississippi River actually flows down Ole Man River.

But in arctic and subarctic areas, rivers typically carry 55 to 65 percent of precipitation falling onto their watersheds. The reason is that permafrost prevents the downward percolation of water and forces it to run off at and very near the ground surface.

One of the consequences of the high runoff is that northern streams are much more prone to flooding than temperate zone rivers. Another is that they have higher eroding and silt-carrying capabilities.

Critical to the control of water runoff in the north is the cover of moss and other vegetation of the tundra, bogs and forests. A thick layer of moss acts much like a sponge laid over the permafrost to slow down the movement of water across the ground surface.

The removal of moss and timber for mining apparently had severe effects on stream flow during goldrush days in Alaska and Canada. It is said that in the course of just a few years miners began to notice faster runoff and less evenness in stream flow. No doubt the removal of overburden also melted some of the permafrost and allowed greater downward percolation of water.

Fortunately, the mosses and forests do recover from the effects of mining, just as they do from the temporary destruction of forest fires. Many areas that old photographs show to be totally barren through the actions of early-day miners now have recovered so well that they show little obvious evidence of man's having been there.