Riparian vegetation structure and floodplain hydrology on dammed vs. undammed rivers: Dolores & Animas Rivers, southwest Colorado

Background/Question/Methods

Regulated flows on western rivers change flood regimes, reduce flow variability, and impact riparian vegetation in many ways. On the dam-controlled Dolores River, the magnitude of peak flows due to snowmelt are significantly lower, or non-existent during drought years like 2012. We have documented vegetation changes below the dam, including declines in the cover of cottonwood woodlands on the floodplain and increases in the cover of willow thickets on the river’s banks.  We hypothesize that these changes are due to a lack of groundwater recharge for the former, and minimal bank scouring for the latter, both as a result of lower peak flows. To test these hypotheses, we make comparisons between long-term study sites on the Dolores River above and below McPhee Dam, and sites on the undammed Animas River.  Here we report the results of four years (2010-2013) of shallow groundwater well data from the Dolores River and two years (2012-2013) of data from the Animas River. Each site contains a transect of three wells to measure groundwater depth. Vegetation cover data have been collected along line-intercept transects at each site. River discharge data are compiled from USGS and Colorado Water Resources stream gauges near the study sites.

Results/Conclusions

In the river bank willow zone, groundwater wells maintain water levels that mirror in-channel flows, and have not gone dry during our study period.  Willow cover on point bars is high at all sites.  The main difference is that the two undammed sites have a significant zone of relatively bare ground at the outer limits of point bars that is missing at the dammed sites, associated with the lack of scouring by high flows and year-round dam release stream flows. In contrast, wells in the cottonwood zone farther into the floodplain did go dry during the extreme drought of 2012, even on the undammed Animas River, though not upstream of the dam on the Dolores. Our long-term data set from one Dolores River site indicates that canopy cover of the dominant cottonwoods has declined over time (48% in 1995, 19% in 2003), especially in the wake of severe drought, which corroborates results of other workers (Coble & Kolb 2012). Cottonwood cover is significantly higher at sites that are not dammed (p=0.02).  In 2012, as was seen in the intense drought of 2000/2003, floodplain cottonwood woodlands were severely stressed by low water table levels associated with low dam release stream flows.