The Wandering Willow: Coyote Willow Growth Along the Dolores River, CO 
 Hannah Holm1*, Dr. Cynthia Dott1

1Department of Biology, Fort Lewis College, Durango, CO, USA

Over the last two decades, the western United States has been experiencing drought on historic levels. This drought has impacted dam-regulated river systems throughout the country. On the Dolores River in Colorado, over-allocation of water resources with McPhee Dam have led to artificial drought and overgrowth of woody vegetation. The overgrowth of the coyote willow (Salix exigua) has been observed but the causes have not been determined. This increase in willow growth has caused channel narrowing and habitat simplification. We hypothesized that consistent low flows caused by drought conditions may have caused the increase in willow establishment seen along the lower Dolores River, and we set out to test this idea by exploring what combination of hydrologic and flow variables were the best predictors of willow establishment. Three sites with two to three sampling stands each were identified along the lower Dolores River below McPhee Dam to collect samples of willow stems. The annual growth rings in each sample were counted to estimate the year of establishment, and to look for correlations with environmental variables. The Palmer Drought Severity Index (PDSI) was used as a proxy for climatic conditions, and river discharge data were collected from the USGS stream gauge at Bedrock, Colorado. Dendrochronology data from willow samples showed establishment years ranging from 1999 to 2020. With this information, drought severity and flow data were collected from 1998 to 2023 to see if flow and drought severity had any impact in the year before, of, and after establishment of each sample. Using Generalized Linear Models (GLMs) with a Poisson distribution, we found that the most significant predictors of willow stem establishment were low minimum flows the year of establishment and a less negative drought severity index score. This result was interesting, as willows are not drought-tolerant due to dependence on water to keep their root systems wet. However, years with lower minimum flows provide more habitat area for willow spread, while less dry years may favor seed germination on those exposed sites. The better we can understand these willow establishment patterns, the more successful we will be in habitat restoration and management efforts in this unique river system. Habitat restoration could involve cutting back larger willows to lessen the channel narrowing effect. This would also help get the plants out of the water, creating a more suitable habitat for fish and other aquatic organisms.