Streams in urban and urbanizing watershed are impacted by altered watershed runoff hydrology and sediment yields, floodplain modifications, and constrained channel planform. One morphological response to these urbanization impacts is the degradation of pool-riffle sequences. Pools and riffles are fundamental mesohabitat units where many lotic biota have evolved to occupy preferentially. Restoring self-maintaining pool-riffle structures is essential to the ecological rehabilitation of urban streams when lost. However restoring these structures can be problematic after a stream has been straightened from prior land development, and current civil infrastructure preventing channel re-meandering. Project goals included: 1) developing a conceptual restoration design by applying geomorphic and three-and two-dimensional hydraulic principles, focusing on flow acceleration and deceleration zones to maintain pool-riffle structures in a straight channel, 2) using River2D, a two-dimensional hydrodynamic model as an ecohydraulic tool to design a pilot project on Beaver Creek, Knox County, Tennessee, and 3) constructing the designed project with follow-up preliminary monitoring and assessment. Ecological information was integrated into the design process from pre-construction monitoring of fish communities, and application of the aquatic habitat module in River2D, utilizing habitat suitability curves (velocity, depth, and substrate) for three fish species. River2D also provided estimates on spatially-distributed shear velocities aiding the design process, which were used for examining channel and bank stability, and placement of root wads. Construction of four poolriffle structures on Beaver Creek was completed in March 2012. A geomorphic survey was completed in April 2013, in which the constructed riffle structures have remained stable even with the project site experiencing eight bankfull events. Post-construction monitoring has shown 3 that the unique design for planform-constrained urban channels has promise for increasing hydraulic habitat diversity and improving ecological integrity in these stressed environments.
Episodic acidification of surface waters has been observed in the Great Smoky MountainsNational Park, similar to other forested watersheds with base-poor bedrock in the eastern US receiving acids from atmospheric deposition. Three remote, forested, high-elevation streams were selected in the Little Pigeon River watershed for study; two of which brook trout have extirpated, and believed to have resulted from severe acidity during stormflows. This research characterized stream chemistry during episodes in order to better understand potential factors that contribute to rapid drops in pH and acid neutralizing capacity (ANC) during stormflows. Autosamplers initialized by sondes, collected samples during storm events for analysis of pH, ANC, cations, and anions over a 15-month period. ANC and pH depressions, and increased concentrations in sulfate, nitrate, and organic acids were observed for all storms at each study site. ANC contribution analysis indicated sulfate was the strongest contributor to ANC depressions, but nitrate, cation dilution, and organic acids were also significant in some cases. Acidic deposition appears to be the primary source of episodic acidification, supported also by the finding that larger stormflows preceded by long, dry periods resulted in significantly larger pH depressions. It appears stream acidification episodes may be driven by acid deposition. However, this study documents the variability of several ion contributors to observed stormflow ANC depressions illustrating the spatial and temporal complexity of watershed processes that influence this phenomenon.
Episodic stream acidification from atmospheric deposition is suspected to detrimentally impact native southern brook trout (Salvelinus fontinalis) in Great Smoky Mountains National Park (GRSM) headwater streams. To test the hypothesis that episodes of stream acidification cause physiological distress to native trout, caged fish at three sites were exposed to acid episodes during in situ bioassays conducted in June 2006 and March 2007. Stream pH decreased (>0.7 pH units) and total dissolved aluminum (Al(TD)) increased (>175 microg/L) at all three sites during acid episodes in both bioassays. Whole-body sodium concentrations were significantly reduced (10-20%) following the acid episodes, when preceding 24-h mean pH values of 4.88, 5.09, and 4.87 and corresponding 24-h time-weighted average Al(TD) concentrations of 210, 202, and 202 microg/L were observed. Lower whole-body sodium concentrations were correlated with elevated H+ and Al(TD) concentrations. Loss of sodium ions in native southern brook trout was consistent with physiological distress resulting from acid exposure reported in salmonids in other investigations. Further research is necessary to conclude whether acid episodes are responsible for extirpation of brook trout from headwater streams in the GRSM.
Relationships between stream chemistry and elevation, area, Anakeesta geology, soil properties, and dominant vegetation were evaluated to identify the influence of basin characteristics on baseflow and stormflow chemistry in eight streams of the Great Smoky Mountains National Park. Statistical analyses were employed to determine differences between baseflow and stormflow chemistry, and relate basin‐scale factors governing local chemical processes to stream chemistry. Following precipitation events, stream pH was reduced and aluminium concentrations increased, while the response of acid neutralizing capacity (ANC), nitrate, sulfate, and base cations varied. Several basin characteristics were highly correlated with each other, demonstrating the interrelatedness of topographical, geological, soil, and vegetative parameters. These interrelated basin factors uniquely influenced acidification response in these streams. Streams in higher‐elevation basins (>975 m) had significantly lower pH, ANC, sodium, and silicon and higher nitrate concentrations (p < 0.05). Streams in smaller basins (<10 km2) had significantly lower nitrate, sodium, magnesium, silicon, and base cation concentrations. In stormflow, streams in basins with Anakeesta geology (>10%) had significantly lower pH and sodium concentrations, and higher aluminium concentrations. Chemical and physical soil characteristics and dominant overstory vegetation in basins were more strongly correlated with baseflow and stormflow chemical constituents than topographical and geological basin factors. Saturated hydraulic conductivity, of all the soil parameters, was most related to concentrations of stormflow constituents. Basins with higher average hydraulic conductivities were associated with lower stream pH, ANC, and base cation concentrations, and higher nitrate and sulfate concentrations. These results emphasize the importance of soil and geological properties influencing stream chemistry and promote the prioritization of management strategies for aquatic resources. Copyright © 2012 John Wiley & Sons, Ltd.
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