Session B8- Evaluating Diadromous Fish Passage at Lower Shannock Falls Dam Removal and Nature-like Weir Installation, Pawcatuck River, Rhode Island

The Lower Shannock Falls dam, a defunct, run-of-the river timber-crib dam situated on bedrock ledge, and located on the Pawcatuck River in southwestern Rhode Island, was removed in summer 2010. The target species for this fish passage restoration include river herring, American shad, American eel, and Atlantic salmon, as well as resident trout. Besides dam removal, additional construction work activities were completed for purposes of improving target species passage efficiency: three nature-like stone weirs were constructed for purposes of providing backwatering effects downstream of the dam removal; and bedrock ledge was modified to eliminate significant hydraulic drops. During construction, test flows were released through the weirs and restored river reach to evaluate hydraulics of the passage site. Based on test flows, several large boulders were installed as adaptive measures to improve upon site hydraulics. Post-construction velocity and depth measurements were completed in fall 2010 and spring 2011 to evaluate passage performance. This presentation will compare the results of the field monitoring with the as-built features and HEC-RAS and Fish Flow modeled hydraulic conditions for the site. Discussion will also include upstream passage conditions that resulted from impoundment drawdown and channel restoration. This project exemplifies Northeastern U.S. dam removals where underlying bedrock may have an effect on passage efficiency, and in-channel work activities may be beneficial to improving upon historic passage conditions. The project is being evaluated as a case study site for a web-based fish passage manual being developed jointly by NOAA-USGS-USFWS, scheduled for release in 2012