Community | Conservation | Science

Draining pond in Maine has good unintended results

By Jeff Reardon

Since 2014, TU’s Merrymeeting Bay Chapter has been focused on brook trout in coastal streams, at least some of which migrate downstream to saltwater for part of their life.  

They’ve been working with me to document brook trout populations in previously un-surveyed streams by angling and electrofishing surveys; monitor water temperatures with long-term data loggers; and, with the owners of three local dams, re-open Frost Gully Brook to allow migration from the headwaters to the estuary. 

We’d been thinking that fish passage was the primary reason for these dam removals, and we still do. But, this past summer, the temperature monitoring revealed that the water temperature benefits of dam removal may be even more important.  

To complete surveys and plan for the dam removals, we worked with Maine Water, who used to operate all three dams for municipal supply, and the Freeport Conservation Trust, which now owns two of the three dams, to drain the impoundment of the largest reservoir.  

We opened a valve to drain the impoundment. When our survey was complete, the valve was no longer operable — it had not been maintained in more than 30 years — so the impoundment remained drained through the summer and fall. 

Because of the Merrymeeting Bay Chapter has multiple monitors above and below the dams, this provided an unintended experiment to tell us what impact removing the largest, warmest impoundment would be.   

Our 2018 monitoring showed that average daily water temperatures at the base of the Burr Pond Dam were 22-24 degrees Celsius (71-75 F) in late July and August, with daily peaks as high as 27 C (81 F).  

Not surprisingly, we found very few trout in angling surveys in that impoundment, and largemouth bass were common.   

In 2019, with the dam still in the place, but the impoundment drawn down, the average daily temperature never rose above 20 C (68 F), with much lower daily fluctuations and a peak water temperature of 21 C (70 F). This cooling influence was notable at two additional stations as much as a half mile downstream. 

Our monitoring results provide strong evidence that the removal of the three dams in this stream reach will restore ideal water temperatures for brook trout to more than a half mile of river that is currently unsuitable. 

Jeff Reardon oversees Trout Unlimited’s Maine Brook Trout Project. He can be reached at jeff.reardon@tu.com.