One fish kill stretched five miles down Washington’s Omak Creek, and wiped out more than 10,000 trout and steelhead. Another fish kill hit five miles of Colorado’s Mancos River. Others hit several Oregon streams. The cause?

Fire retardants dropped by airplanes, as federal agencies battled wildfires during the past three years.

The plume of chemicals reaches streams in “less than one-tenth of 1 percent of all the retardant drops,” estimates Alice Forbes, at the National Interagency Fire Center in Boise, Idaho.

But with 15 to 18 million gallons dropped mostly by federal agencies in an average year, and as much as 44 million gallons dropped in a bad fire year, even the small percentage ending up in streams is too much, says Andy Stahl of Forest Service Employees for Environmental Ethics.

U.S. Forest Service rules say retardants should not be dropped within 300 feet of streams, but it happens — sometimes by accident, and sometimes when it’s necessary during firefighting. “Pilots drop retardants in the worst conditions (for accuracy) — low-level flying, smoke, wind,” Stahl says.

Retardants can kill aquatic life because they contain chemicals including fertilizer compounds. When the leading brand, Fire-Trol, is exposed to sunlight and water, it releases cyanide. The government is studying the problem, and by 2005 will likely stop using any retardant that releases cyanide, Forbes says.

Stahl’s group wants a full-scale environmental impact study of retardants and other tools of the war on wildfires, and has filed notice it will sue the feds in July, trying to force firefighters to be more careful.