The Beaver-Meadow Complex

by Ronald Ives (from Journal of Geomorphology)

This is a simple explanatory article, short on data, which tries to correct the widespread impression that the many "large wet meadows occupying the floors of all major and most minor valleys" in the Rocky Mountains are "silted-up glacial lakes." Ives shows that these meadows have gradients, many exceeding 200 feet to the mile. Glacial lakes would not leave gradients. The mountain meadows have beds of "gravels, sands, and clays" and only a small amount of the peat to be expected in a glacial lake. In addition, "separating the various quantity of sedimentary material is a vertical 'honeycomb' of partially decayed woody material, which has the same general arrangement and form as the modern surface system of beaver dams, and is composed of materials in the same size range." In a map of the Colorado River headwaters, Ives shades in the many beaver meadows.

He then describes how the beavers create "an extensive artificial environment of its own making." Indeed until 10,000 years ago "the surficial changes wrought by the beaver exceeded in magnitude those produced by man." Ives suggests the quality of that work leads to the permant nature of the changes beavers wrought. "Although beaver dams appear quite fragile, their construction makes them resistant to most natural destructive influences, such as ice thrust."

Evidently Rocky Mountain beavers are largely dependent on aspens, and perhaps, because they have so often been studied, not a few general guides on beavers insist that the aspen is its favorite food and that without aspen the beavers move on. (If this were the actual case there would not be any beavers within miles of where I can see them every night.) Ives argues that beavers effect the "perpetuation" of their artificial environment:

It is commonly stated by zoologists that the beaver exhausts the aspen supply in the immediate vicinity of his pond in a few years and then migrates to some other area. This statement, while strictly true, is decidedly misleading. Beaver, after exhausting the aspen supply adjacent to their pond, do migrate, but the new pond, in most instances, is only a few hundred feet away from the abandoned site, and usually is just upstream from the old pond. The zoologist's explanation leaves the general impression that the beaver mines out his environment. Actually, through the interaction of a number of biotic and geologic factors, he farms it.

The beaver is able to "farm" these areas because aspen are the first tree to appear in the normal forest succession. So when beavers abandon what has become grassland and dense brush, they are leaving an area ripe for the growth of aspen.

Ives does not suggest that the effect beavers' work is limited to the mountain meadows. "Ancient beaver dams, buried under thick deposits of sheetflood debris, have been found in many of the more arid valleys of the Colorado mountain region."

Finally, Ives credits the "false senility" that beavers give to mountain streams with being "effective elements in the aggradation of the valley floors."

Bob Arnebeck

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