Post-Restoration Evaluation of Two Urban Streams: Austin, Texas USA

Rapid urban growth of Austin, Texas, has resulted in significant alteration of the surface characteristics of the Colorado River Watershed. These changes have increased runoff and accelerated erosion within stream channels. To minimize the threat of stream erosion to real estate and infrastructure, the City of Austin began restoring unstable channel reaches by placing rock armor along banks, constructing rock grade controls, and planting riparian vegetation. Since the late 1990s, approximately thirty channel reaches have been restored in the Austin area. Considerable discussion is occurring regarding the true impact of stream restoration; however, few studies have conducted post-project evaluations to assess the impact of restoration efforts. Because a sufficient time period for stabilization to occur has passed, we believe these projects could be assessed for the temporal impact of restoration on these streams.

We conducted a channel stability assessment and used repeat ground photography to evaluate the condition of restored and present “natural” reaches of Waller Creek and Tannehill Branch. The “natural” reaches served as ergodic surrogates for temporal channel development of the restored reaches. Variables of channel morphology analyzed included riparian vegetation cover, vertical stability, scour/deposition potential, and bed sediment composition. From our analysis, restoration enlarged stream channels, decreased bank height ratios and reduced flood prone width. Bed sediment analysis suggested that pools contain a higher percentage of fines whereas riffles sediments are coarser in restored reaches than pre-restoration reaches. Visual examination of ground photographs and the Pfankuch channel stability evaluation indicate that restoration increased vegetative cover and deposition. Thus, restoration efforts had a positive impact on these two streams. Data from channel stability assessments provide the basis upon which longer-term monitoring and evaluation can be conducted. Knowledge gained from long-term monitoring is important for improving the effectiveness of current and future restoration projects.