Document: GAB-3-76-11

Delayed effects of flood control on a flood-dependent riparian forest.

KATZ, G.L.* ¹ and J.M.FRIEDMAN ²

University of Colorado, Boulder, CO 80309 ¹
U.S. Geological Survey, Ft. Collins, CO 80525 ²

Abstract:
In the southern Great Plains, the dominant native riparian tree, plains cottonwood (Populus deltoides subsp. monilifera), depends more on physical disturbance for reproduction than does the exotic Russian olive (Elaeagnus angustifolia). We hypothesized that flood control should favor the exotic species. We compared the riparian forest along the unregulated Arikaree River, with the forest on the regulated S. Fork Republican River upstream and downstream of Bonny Reservoir in eastern Colorado, USA. Completed in 1951, Bonny Dam significantly reduces downstream peak and mean flows. We related stem density of both tree species to hydraulic position, and collected increment cores from 694 trees along 9 valley cross sections. In all 3 reaches, cottonwood stands originated during an extended episode of channel narrowing following the 1935 flood of record. There is essentially no current cottonwood recruitment, since new seedlings are restricted to a narrow frequently disturbed zone within the active channel where survival is unlikely. Compared to cottonwood, Russian olive populations are younger and more uneven-aged, indicating ongoing recruitment under current environmental conditions. Bonny Dam has not yet affected downstream adult cottonwood age structure because no flood large enough to promote significant establishment has occurred since dam construction.

Keywords: riparian forests, Colorado, biological invasion, dam effects

This abstract is being presented at: 10:15 AM in session:
Oral Session #20: Riparian Ecology.