This study took place in one well developed stand of riparian trees and shrubs that appeared to be even-aged along the San Juan River near Chinle Wash in southern Utah. The purpose of the study was to examine beaver (Castor canadensis) foraging behavior and their impacts on cottonwoods (Populus fremontii). The foraging models predict that as the distance from the river's edge increased beavers should forage more selectively by cutting fewer branches per plant. In this study, at a distance of only 3.5 m from the river's edge, 71% of the branches were cut by beavers, whereas at a distance of 73 m only 20% of the branches were cut. This result supports the prediction of foraging models. Large branches were favored at all distances, which differs from patterns observed in previous studies of beaver foraging. This difference, however, is expected and supports Schoener's (1979) predictions which are based on differences between provisioning costs and item size. The selective foraging of beavers is responsible for the observed differences in P. fremontii growth form. The more plants were fed upon by beavers the shrubbier their growth form became. Plants with the most branches also suffered the greatest removal of branches. At this study site the foraging of beavers did not kill the plants, but their pruning results in the production of a dense hedge.
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