CONCORD, N.H. (AP) – Even rivers need prescriptions to stay healthy.

Small, seasonal changes to the volume of water flowing through dams means the difference between life and death for rare and endangered species living in the rivers of New Hampshire and Vermont, says Doug Bechtel, a director of conservation science for the Nature Conservancy.

He’s thinking about the dwarf wedge mussel, a federal endangered species that lives in the mud of the Ashuelot River. Dwarf wedge mussels were in trouble a few years ago when low water levels left them perched in open air, where they either were drying out or being devoured by muskrats. But dam managers, with help from the U.S. Fish and Wildlife Service, tweaked the flow “prescription” to ensure the mussels would be submerged.

“It’s mostly about timing,” said Bechtel. “Those kinds of minor changes can have a huge impact.”

By the end of next year, Bechtel and other scientists from the Nature Conservancy and the Army Corps of Engineers hope to formulate flow “prescriptions” for the Ball Mountain and Townshend dams in Vermont’s West River, as well as the Otter Brook and Surry Mountain dams New Hampshire’s Ashuelot River. The West and Ashuelot flow into the Connecticut River.

Officials from both groups cemented the partnership on Thursday in Burlington, Vt., where they signed an agreement to continue the study. Chapters of the Nature Conservancy and the Army Corps are embarking on similar partnerships across the country.

“It … emphasizes the relationship that we’ve had in the past and it points us toward more partnerships in the future,” said Col. Curtis Thalken, New England district Commander of the corps.

Part of the study will include a census of species, living downstream of dams, that give clues to a river’s general health, Bechtel said.

“We’ve got a list of what lives there, but which are the ones that are going to tell us if the river is healthy, and if the flow coming through the dam is correct?”