Greenhouse and field studies were used to assess the effects of a cottonwood leaf beetle (Chrysomela scripta) outbreak on growth and survivorship of Fremont cottonwood (Populus fremontii) along a segment of the Yampa River in northwestern Colorado. Exposure of greenhouse-reared seedlings to either 1 or 3 larvae reduced survivorship and the shoot and root masses of surviving seedlings. In the field sapling survivorship in unmanipulated, naturally established patches of saplings was inversely related to the intensity of beetle damage noted the previous year in both 1998–1999 and 1999–2000. Floodplain saplings treated with the insecticide esfenvalerate during two growing seasons added more height and radial growth than untreated saplings. Field observations in riparian stands along the Yampa River indicated that beetles preferentially oviposited and fed on cottonwood seedlings over saplings and mature trees during late summer. Taken together, the greenhouse and field studies suggest herbivory by C. scripta is a major determinant of Fremont cottonwood distribution and abundance at the Yampa River site. Repeated surveys detected few or no beetles in stands along the neighboring but regulated Green River. We hypothesize that flow regulation, by reducing P. fremontii seedling abundance, precludes C. scripta from reaching outbreak status along the Green River.