Four-year map finds 6% of lower-48 river length both healthy and protected

Add the two healthy groups together and about 19% of the country's stream length is in good condition ^[1]. Only about a third of that, 6 points of the 19, is meaningfully protected ^[2]. The other 13% is in good shape with no real safeguards ^[3]. On land, the healthiest places tend to be the protected ones. For rivers, the study found, health says almost nothing about whether anyone is looking after it ^[12].

The grades start from habitat and flow ^[5]. Each watershed's four scores were laid over 91 existing assessments of where freshwater life is concentrated, and that overlay is where the biology comes in ^[6]. Dozens of scientists and partners from 44 states then adjusted the measures for regions as different as the humid East, the arid Great Plains and the Southwest deserts ^[7]. The result is a grade of each river's ability to sustain aquatic life in a warming climate ^[8].

The denominators need care. The assessment graded every river and stream in the lower 48 ^[8], and it maps one-third of the nation's stream distances ^[9]. More than half of that mapped length came out resilient ^[10]. The 6% and 13% figures are shares of the country's stream distances as a whole ^[1][3].

Mary Khoury, a freshwater ecologist at The Nature Conservancy, stated the goal directly. "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," she said ^[4]. For the restorable group, the study's examples of what a river needs are fixes for runoff from a farm or an obstructing dam ^[11]. More than 150,000 dams have fragmented the country's waterways, cutting fish off from spawning grounds and cold-water refuges ^[15].

The thing this doesn't tell a funder is what any repair costs. The map orders rivers by what they could support. Someone weighing a dam removal against a runoff project still has to bring the prices.

The resilient network already includes long, unbroken stretches ^[16]. The Klamath River in Oregon and California lost its lower dams in the largest removal project in U.S. history, and the Delaware runs free of barriers along its main stem from New York's Catskill Mountains to the sea ^[16]. "A resilient system is one that gives species options and environments in which to adapt," Khoury said ^[17].

Rafael Schmitt, a hydrologist at the University of California, Santa Barbara who was not involved in the work, suggested pairing the map with regulatory dates. "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," he said ^[18].