That finding is tempered by growing signs that the nation's rivers are getting dirtier overall - after decades of getting cleaner, the same study notes. From about 1973 to 1998, rivers and lakes in the United States were getting cleaner, but that's now reversed itself, according to American Rivers, which helped organize the new study.
Examples: More than a third of US rivers are listed as polluted or impaired. Extinction rates of freshwater fish are about five times the level of land animals. Withdrawals of freshwater from rivers for agricultural and other uses is so extreme in some regions that the rivers no longer reach the ocean all year long, the study says.
Adding to the challenge: aging sewage-treatment facilities that, without a renovation costing $320 billion to $450 billion, could in 2016 return US rivers to the way they looked - and smelled - in the 1970s, according to a 2002 report from the Environmental Protection Agency.
Water quality is now beginning to decline for several reasons, Mr. Fahlund says. The Clean Water Act did a good job initially of cutting pollution from "point sources," the big reason for improvements over 30 years. But at the same time, nonpoint sources - runoff from parking lots, streets, agriculture - were getting worse. Today, nonpoint pollution has in many places surpassed those earlier gains.
On the other hand, the threat of costly and onerous environmental penalties has driven the increase in river and stream restoration, Fahlund says. Federal and state funds have been ladled out for larger projects. Smaller, private "mitigation funds" - compensation paid by developers in exchange for environmental degradation from their projects - have financed the huge growth in river restoration.
In the Southwest, much of the focus is on saving endangered species like the humpback chub, while in Chesapeake Bay the focus is on cleaning up excess sediment and nutrient pollution. In the Southeast, meanwhile, developer "mitigation credits" are the major propellant. Slightly more than half of the projects - about 20,000 - are in the Pacific Northwest.
One of those projects can be found in an isolated region of Idaho. For millenniums, spawning chinook salmon found their way up the Columbia and its tributary, the Red River, a modest 50-foot-wide tributary that once wound its way in serpentine coils across mountain meadows of north central Idaho.