POST-RESTORATION EVALUATION OF TWO URBAN STREAMS: AUSTIN, TEXAS USA

Rapid urban growth of Austin, Texas, resulted in significant changes in the surface characteristics of the Colorado River Basin watershed. These changes increased the runoff and accelerated erosion in the stream channels. By 1995, the City of Austin had identified 947 cases of localized stream erosion, with 160 channel reaches classified as unstable. To minimize the threat of stream erosion to real estate, the City of Austin began restoring unstable channel reaches through the addition of rock armor on the banks, construction of limestone rock grade controls, and planting of riparian vegetation. Since the late 1990s, approximately 30 channel reaches have been restored. Although much attention is given to the “remarkable” positive impact of stream restoration, one has to ask: what is the impact of restoration efforts on the stability of streams? Because it has been several years since steps were taken to restore these streams, these projects can be assessed for the temporal impact on streams.

We studied the restored and natural reaches of two of these streams. The natural reaches served as ergodic surrogates by substituting space for time. The first stream is ~ 11 km long with a restored reach that is 0.1 km long. The second stream is also ~ 11 km in length with 3 restored reaches ranging from 0.1 km to 0.6 km in length. Both streams flow over a limestone bedrock and have perennial flows. We used Rosgen's methodology of channel stability assessment to determine the status of the two streams. We analyzed the variables of channel morphology including bed sediment, riparian vegetation cover, vertical and lateral stability, and scour/deposition potential. From our analysis, restoration has enlarged the channel capacity and increased the width to depth ratio. Bed sediment analysis revealed a higher percentage of finer sediment occuring in restored reaches than natural reaches. Thus, restoration efforts are working on these two streams. Data from the assessments of stream channel stability provide the basis upon which longer-term monitoring and evaluation can be conducted.