A new map points out U.S. rivers most worth protecting

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. 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. 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. 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. Dams leave the clearest mark on the country’s rivers, carving them into more than 9,000 separate pieces.