The effects of flow regulation on the population biology and ecology of beavers in northwestern Colorado

My research assessed the effects of flow regulation on beaver ( Castor canadensis) populations and on the interaction between beavers and two plant species, sandbar willow (Salix exigua) and Fremont cottonwood (Populus deltoides spp. wislizenii). I gathered comparative data on two 6th-order alluvial rivers, the free-flowing Yampa River and flow-regulated Green River in Northwest Colorado from 1997–2000.; I assessed the effects of flow regulation on the demography of beavers by comparing the density, survival, home range and body size of bank-dwelling beavers. On the Green River beaver density was higher, adult and sub-adult beavers were in better condition, and mortality due to predation was lower. I attributed the improved habitat quality on the Green River to a greater availability of willow. I quantified the abundance and density of sandbar willow and the amount of willow taken by beavers on alluvial sections of each river. On the Green River, willow has shifted its distribution from a primarily bank-oriented to a primarily island-oriented species. The total area of willow was similar on each river, though on the Green River patches were smaller, more numerous, contained a lower density of stems, and stems grew faster. Beavers ate 2–3 times more willow on the Green River than on the Yampa River. Flow regulation increased the availability of willow by altering its distribution.; I quantified the abundance and density of cottonwood and the survival of cottonwood saplings due to beaver herbivory. Flow regulation has caused a decrease in number of cottonwood patches per km of river, area of patches per km, and average stem density within cottonwood patches on the Green River. Controlled flooding increased the rate of foraging on the Green River. I suggest regulation has magnified the beaver's impact on cottonwood through interrelated effects on plant spatial distribution and cottonwood abundance.