The detailed assessment suggests what would have the biggest conservation bang for the buck

First, the bad news: Of all the rivers in the contiguous United States, only 6 percent are both in healthy condition and meaningfully safeguarded. The good news is that many more may be just one repair away from thriving.

A new map highlights where conservation money and attention would do the most good as the climate warms, researchers report September 16 in PLOS One. The effort is the most comprehensive to date, assessing the health of the lower 48 states’ rivers and streams.

“We can now point at a map for the first time and say, these are the rivers most capable of sustaining diversity of aquatic life, and this is what each one needs,” says Mary Khoury, a freshwater ecologist at The Nature Conservancy who is based in Chicago.

For decades, conservation in the United States has been organized primarily around land. Scientists know for the most part which forests are intact and who owns what. Rivers have never gotten that treatment, even as more than 150,000 dams have fragmented the country’s waterways — cutting fish off from spawning grounds and cold-water refuges — and climate change has altered the temperatures and flows these animals depend on.

The new assessment, which took four years, maps one-third of the nation’s stream distances. Each small watershed was rated on four measures: how connected its river network is, meaning whether animals including fish and mussels can freely travel along it; the condition of its floodplain and water quality; how much water it has; and how much its flows have been altered. Those ratings were laid over 91 existing assessments of where freshwater life is concentrated. Dozens of scientists and partners from 44 states adjusted the measures for river systems in regions as different as the humid East, the arid Great Plains and the deserts of the Southwest.

River resilience

A four-year assessment graded every river and stream in the lower 48 for its ability to sustain aquatic life in a warming climate. The healthiest ones cluster along the Pacific Coast, in the northern Rockies and across the Southeast. The most vulnerable sit in the Southwest deserts, the Great Plains and southern New England.

The Nature ConservancyThe Nature Conservancy

Rivers doing well on all four criteria — more than half of the mapped stream distances — came out as resilient. The rest score near average, and were classed as restorable, still functional but held back by a problem, such as runoff from a farm or an obstructing dam, that one intervention could solve.

Together the two categories form what the authors call a freshwater resilient and connected network, a shopping list of what the country would need to look after to keep its freshwater life going. “A resilient system is one that gives species options and environments in which to adapt,” Khoury says.

Dams leave the clearest mark on the country’s rivers, carving them into more than 9,000 separate pieces. The study maps where reconnecting rivers would pay off most. Among the long unbroken stretches within the resilient system are Oregon and California’s Klamath River, whose lower dams came out in the largest removal project in U.S. history, and the Delaware River, which runs free of barriers along its main stem from New York’s Catskill Mountains to the sea.

“I can see this being overlaid in the future, for example, with where dam licenses will expire, in order to find the most effective restoration opportunities,” says Rafael Schmitt, a hydrologist at the University of California, Santa Barbara who was not involved in the work.

Unlike on land, the study found, a river’s health says almost nothing about whether anyone is looking after it. On land, the healthiest places tend to be the protected ones. In water, however, 13 percent of the country’s stream distances are in good shape with no real safeguards, roughly twice the 6 percent that is both healthy and protected.

The unprotected healthy rivers are the ones most at risk of slipping, because nothing keeps them in that condition. Locking in protection is harder for a river than for a forest. A designated preserve can hold a kilometer of stream while the water arriving in it may drain 100 kilometers of farm fields, dams and subdivisions. “It matters what’s going on upstream of that protected area,” Khoury says.