How is stream temperature being effected by small surface release impoundments across Massachusetts?

Impoundments offer a multitude of societal benefits (e.g., water supply, recreation, flood protection), but often adversely affect downstream aquatic ecosystems. Impoundment management has largely focused on minimizing alteration to the natural flow regime, underappreciating the impact on temperature. Impoundments have been identified as one of the leading causes of altered thermal regimes in streams; however, more research is needed to determine the longitudinal extent of this temperature alteration, especially beneath small surface release impoundments.

This study quantifies the distance of stream temperature alteration downstream of small impoundments and examines environmental factors controlling the timing and magnitude of this recovery. We selected 21 stream sites across Massachusetts encompassing three impoundment types: water supply impoundments, run of river impoundments, and beaver dams. Downstream of each impoundment, we installed 3–6 temperature data loggers. The first logger was installed as close as possible to the impounded area and the others were spread over varying downstream reach lengths. Temperature was recorded every 15 minutes from May to October 2014.

Differences in monthly mean, maximum, and minimum temperatures between upstream and downstream loggers will be used to compute temperature "decay" slopes and recovery distances. We will assess the effect of streamflow, impounded surface area, catchment relief, forest cover, impervious land cover, and road density on the longitudinal extent of thermal alteration downstream of impoundments. The results from this study may be used to improve the accuracy of stream temperature models and to assess potential for climate resilience associated with impoundment removals.