EVALUATING RESTORATION SUCCESS FOR THE LITTLE THOMPSON RIVER (CO) THROUGH A FUNCTIONAL TRAITS FRAMEWORK

In September 2013, the Colorado Front Range experienced an extreme rainfall event causing mass flooding in the region. Many Front Range rivers were impacted and subsequently restored. In the restoration industry, there is little standard in approaches and even less precedence for evaluating project success. We are developing an assessment framework incorporating both ecological and geomorphic variables in addition to project-specific objectives, applied to the Little Thompson River. We integrate high-resolution topographic and functional traits vegetation surveys at both the restored reach and an upstream reference reach, with the ultimate goal of contributing to the knowledge of best practices in river restoration. At the restored site, a step-pool/pool-riffle morphology design was implemented, (2.4% slope and 112 km² drainage area), preliminary analysis from reoccupied as-built cross-sections suggests the designed channel is larger than expected for the ~two-year flow. Furthermore, grain-size analysis indicates a coarse-tailed distribution (D₅₀ = 55 mm and D₉₀= 128 mm) and a steeper slope than expected, due to road reconstruction. We predict typical flows will not connect to the floodplain, nor mobilize bed material—two design objectives. Vegetation surveys indicate of the five woody vegetation types planted, only willows are thriving, comprising 72% of vegetation present. In contrast, 5 years after project completion, 22% of living woody vegetation recruited naturally, consisting predominantly of cottonwoods that require flood disturbance for recruitment. The reference reach has a denser and more diverse plant community, despite significant riparian zone erosion in 2013. We used an ANOVA and Tukey’s HSD to compare plant height, basal diameter, distance from, and elevation above the thalweg by species and found a significant difference (p-values <0.05) in elevation above the thalweg for willows; the restored willows are higher. We hypothesize this is due to the oversized constructed channel that may result in vegetation being disconnected from typical flows with potential implications for future vegetation-river interactions. Our results raise questions about a secondary human disturbance and the efficacy of both channel construction and revegetation efforts.