The West Fork Little Bear Creek in northern Idaho winds through sloping hills and Palouse Prairie farmland on its way to the Potlatch River. The cool, shaded stream seems like typical steelhead habitat.

But just above a narrow basalt canyon sits a wastewater treatment plant, which handles 110,000 gallons of sewage and other municipal waste from the town of Troy each day. The treated water -- still loaded with ammonium and phosphorus -- is discharged into the creek. As a result, its downstream water quality falls below certain state and federal standards; some pollutants have been found at levels the Environmental Protection Agency considers toxic for fish.

And yet wild steelhead are thriving in the substandard water. The creek has the highest density of juvenile steelhead of any surveyed habitat in the entire Potlatch River watershed, where wild steelhead are listed as threatened under the Endangered Species Act. The treatment plant, it turns out, may be filling an ecological void.

"There are no nutrients in the creek," explains Ricardo Sánchez-Murillo, a Ph.D. student in technical hydrology at the University of Idaho. "It seems like this specific system has the potential to absorb this amount of nutrients from the plant."

Sánchez-Murillo was part of a research team that studied the link between the effluent and the creek's steelhead, comparing water quality below the plant to steelhead densities. They found that oxygen levels directly below the plant were below state standards for part of the year, a result of excessive ammonium and phosphorus. But minerals like phosphorous also fertilize the stream, boosting algae growth and the numbers of small insects that steelhead eat. In some streams, too much algae can choke out other stream life. In a nutrient-deprived stream, the right amount of the plant blooms can help the entire system.

The Troy treatment plant operates under an EPA permit that determines how much ammonium and phosphorous can be released each day. Technically, it's been releasing the pollutants at allowable levels. But the plant is overdue for a new permit, and it will likely have to clean up its water to meet state oxygen standards.

And steelhead populations may suffer. "What we're seeing here is an environmental paradox," says Sánchez-Murillo.

Historically, millions of pounds of nutrients were provided by adult steelhead and salmon migrating from the ocean to inland freshwater lakes and streams throughout the Northwest. After salmon return to their natal streams to spawn, they die. The decomposing fish release phosphorus and nitrogen, supporting algae  and  other  plant  life.  The   benefits travel up the food chain from insects to juvenile steelhead to land mammals.

But the number of adult salmon and steelhead that return to Northwest streams to spawn has declined sharply, thanks to dams, pollution, habitat loss and hatchery impacts. Regionally, fish provide just 6 to 7 percent of the fertilizing nutrients they did in the past. In Idaho, a 2003 study found that up to half of streams have unnaturally low levels of phosphorus and nitrogen.

State agencies in Oregon, Washington and Idaho have tried dumping spawned-out salmon and steelhead carcasses from hatcheries into streams to help fertilize ecosystems. But carcass seeding has risks -- dead salmon can carry diseases that threaten already vulnerable fish populations. Agencies have also tried increasing nutrient levels using ground fish pellets, but both options are expensive.

That's why some researchers see wastewater as a potential alternative, though like carcass seeding and pellets, it won't deliver the full ecological benefits of live, returning adult fish, which turn over gravel and manipulate streambeds as they build nests. Wastewater must be treated to remove bacteria and adhere to safety standards, says Erin Brooks, a University of Idaho scientist who leads the West Fork Little Bear Creek research. "(But) in some cases, we are taking more nutrients out of creeks than we should be from a fish-ecology perspective."

Now, Troy, population 840, is wrestling with how to both improve water quality and support steelhead. Not all of the possible options would benefit the fish, and the Endangered Species Act requires that wastewater permits consider impacts to listed species (but not necessarily that all impacts be avoided). The town could upgrade equipment so that fewer nutrients are released, but that means less food for the fish. And effluent could be used for irrigation, except that the discharge also provides critical water volume, another major benefit to fish. Without the treatment plant, chances are the creek would dry out during the hottest summer months. If the city can strike a deal with other water-rights holders, the best option for the fish might be to release water from an upstream reservoir to dilute the pollutants.

Given tight budgets and the diversity of affected streams, such creative solutions may become more common. But Colden Baxter, associate professor of stream ecology at Idaho State University, warns that while wastewater may have value for certain streams, seemingly easy or low-cost substitutes for fish nutrients must be considered cautiously.

"Should that count as mitigation for the loss of salmon and steelhead?" Baxter asks. "I think that is both a scientific question, as well as a question for our culture and ethics."