The Dolores River in southwestern Colorado was one of the last major western rivers to be dammed to provide consistent irrigation water for agriculture. Construction of McPhee Dam was completed in 1984. Field ecology classes from Fort Lewis College in Durango, CO began data collection at a riparian site on the Dolores River below the dam in 1988 under the direction of Dr. Preston Somers. The main goal of this case study was to introduce undergraduate students to field methods and the nature of long-term change in ecosystems. However, a secondary benefit of this project has been the accumulation of a large long-term data set documenting changes in the riparian community since closure of the dam.
In most summers since 1988 students have collected data documenting the abundance and health of tree and shrub species along a permanent transect, in quadrats on the river bank point bar, and in interior forest “meadow” quadrats farther from the river. Starting in 2004, when I took over this portion of the course, we began to expand the scope of the study by adding additional sites along the Dolores River above and below the dam.
Results/Conclusions Over the 20 year span of this study there have been significant declines in the health and abundance of narrowleaf cottonwoods along the permanent transect; an apparent drying of the vegetation in the interior meadows; and a highly significant increase in the abundance of woody vegetation on the point bar. We attribute these changes to a loss of spring flood events due to management of the dam, which carries several unintended consequences. First, the lack of scouring floods has not only failed to remove encroaching willows, but also does not prepare an adequate seed bed for germinating cottonwood seeds, leading to a lack of new cottonwood seedlings. In addition, sediment that was usually carried downstream and deposited on point bars each spring is now trapped in the reservoir. Finally, the stable low flows that now dominate the Dolores River below McPhee Dam are probably unable to recharge the floodplain water table which has lead to an overall drying of the riparian forest. It seems probable that the majority of the floodplain will gradually transition away from native cottonwood-willow woodland into a drier upland forest of juniper and shrubs, causing the riparian zone to narrow in response to this effectively smaller stream.
