The U.S. Geological Survey (USGS) New England Water Science Center has developed a new web application to identify stream reaches that have relatively higher amounts of streamflow during the summer when water levels are lower. These areas have the potential to have high-quality cold-water habitat for endangered Atlantic salmon during a critical time during their lifecycle. This tool was based off of a model published in the journal River Research and Applications last year.

Atlantic salmon are native to the Northeast, and their range previously extended south from Maine to Long Island Sound. Now, only a distinct population of Atlantic salmon live in the Gulf of Maine and enter Maine’s rivers in late spring. Their freshwater range includes all watersheds from the Dennys River south to the Androscoggin River.

A sea-run fish, Atlantic salmon depend on different aquatic habitats throughout their life and are reliant upon access to cold freshwater streams during the summer months when they spawn. Juvenile salmon remain within these systems for several years until they become smolts and return to the sea. As a cold-water species, Atlantic salmon are sensitive to water temperature changes and rely on temperatures under 70 degrees Fahrenheit to survive. Therefore, conserving or restoring stream reaches in Maine that run cold and deep will aid in their recovery.

Listed as endangered in 2009, the U.S. Fish and Wildlife Service (FWS) and the National Oceanic and Atmospheric Administration (NOAA) published a final recovery plan for the Gulf of Maine Atlantic salmon population in 2019 that included the identification and restoration of reaches that had the potential to be favorable cold-water fish habitat. Reaches that have deeper and faster streamflows are typically cooler and contain more dissolved oxygen, making them valuable environments for endangered Atlantic Salmon and other native species.

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In cooperation with NOAA and the Maine Department of Marine Resources (MDMR), the USGS New England Water Science Center created a model to estimate the relative amounts of streamflow in Maine streams during August, typically one of the driest times of the year. High temperatures and minimal rainfall in the summer causes less water to move through river channels. When levels are low, water temperatures rise, and warmer water does not hold onto dissolved oxygen as well as cold water. Therefore, estimating streamflow in reaches when conditions are dry, also known as baseflow, helps resource managers determine which reaches in Maine provide the best environment for Atlantic salmon because they remain deep and cold between precipitation events. Reaches with higher baseflows curb water temperature in summer low-flow periods and improve habitat connectivity because shallow water can strand fish or limit them to certain areas.

“By identifying basin characteristics that explain much of the variability of baseflow in reaches that have a streamflow gage, we are able to better understand baseflow in all reaches, even those without a streamgage,” USGS supervisory hydrologist and lead study author Pamela Lombard said.

The variables used to predict baseflow in the model include the proportion of sand and gravel aquifers and mean July precipitation. The model indicated that reaches that are underlain by sand and gravel aquifers had higher baseflow averages in August than those that did not. This is because these aquifers are typically associated with coarse surficial deposits, or the water-eroded sediments at the bottom of a river, which allow groundwater to infiltrate into river channels more easily. In addition to the higher volume of water, streams fed predominantly by groundwater tend to be cooler than those fed by surface water sources.

In 2022, the researchers applied the August-baseflow model to every reach within a stream network derived from National Hydrography Plus High-Resolution dataset that falls within the Atlantic salmon range to predict which reaches have high baseflows based on the parameters in the model. USGS researchers used this information to publish a Geographic Information Systems (GIS) layer on an web map and on StreamStats for natural resource managers to access when looking to determine the reaches in Maine that have relatively higher amounts of baseflow. With this information, NOAA and MDMR can pinpoint where fish-passage construction, and other conservation efforts, have the best chance of helping Atlantic salmon recovery.