Type of Resources

Dissertation/Thesis

Date Created

8-6-2020

Abstract

In September 2013, the US Colorado Front Range experienced an extreme rainfall event causing mass flooding in the region. Many Front Range rivers were impacted and subsequently restored, especially those adjacent to roadways. Channel restoration practices often lack a standard approach, and evaluating project success is usually lacking. An assessment framework was developed incorporating both ecological and geomorphic variables in addition to project-specific objectives, applied to the Little Thompson River. Topographic and functional traits vegetation surveys were conducted 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, the Forest Service implemented a step-pool/pool-riffle morphology design, (0.024 m/m slope and 112 km2 drainage area). The cross-sectional area of the designed channel is larger than as-built plans for the two-year recurrence interval flow, and not consistently connected to the floodplain. Furthermore, grain-size analysis indicates a coarse-tailed distribution (D50 = 55 mm and D90= 126 mm) and a steeper slope than was designed, due to road reconstruction. Results indicate that typical flows are unable to mobilize the median grain size. The restored reach has a smaller slope (0.013 m/m) and grain size distribution (D50 38 mm = and D90 = 108 mm) than the reference, showing that the reference reach has reached an equilibrium between the channel morphology, grain size distribution, and hydrological regime, whereas the restored reach has not reached an equilibrium. Vegetation surveys indicate that, of the five woody vegetation types planted, only willows are thriving, comprising 72% of vegetation present. In contrast, five years after project completion, 22% of present 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. Using an ANOVA and Tukey’s HSD to compare plant height, basal diameter, distance from the thalweg, and elevation above the thalweg by species, there was a significant difference (p-values

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