IN 2002, DROUGHT led farmers in the Klamath River Basin in Oregon and Northern California to divert river water to their fields. Tens of thousands of coho salmon and other endangered fish died as a result. In 2003, a federal court ruled that river water could no longer be diverted, since doing so violated the Endangered Species Act. The result, in 2004, was mass bankruptcy, as farmers' livelihoods literally dried up.

That's the West Coast story: Look no farther than the Chesapeake Bay watershed for the East Coast version. In a series of stories, The Post has revealed that despite more than a decade's worth of effort, the bay is actually no less polluted now than it was 15 years ago. Its "dead zone" -- the area of the bay in which nothing can live -- actually is expanding. Yet many of the most obvious sources of pollution have already been targeted: Phosphate detergents are banned in the region, and this spring Maryland legislators passed a $2.50 per household "flush tax" designed to raise money for sewage treatment plants. Meanwhile, the largest sources of pollution -- nutrient runoff from some 12,000 farms in Maryland, Virginia, Delaware and Pennsylvania, as well as from cities -- have hardly been tackled. That's because to do so would be complicated and expensive, and might put at risk the income of the region's farmers.

The bicoastal environmental trouble illustrates how difficult the choice can be between clean rivers and "normal" water levels on the one hand, and agriculture and irrigation on the other. In the Klamath Basin, there is no middle road: Either the farmers move away, or the fish die. Nor are there easy choices left in the Chesapeake Bay watershed: Either the farmers cut down drastically on fertilizer use and find some clean but probably expensive means of disposing of manure, or the fish, shellfish and animals that live in the bay will die, and the fishing industry will disappear.

Politicians have only begun to tackle these increasingly tough problems, and not gracefully. Partly reacting to the Klamath Basin farmers' crisis, the House Resources Committee voted last week to pass a set of bills that would give political appointees and local residents a bigger say in how the Endangered Species Act is enforced. Yet tilting the process in favor of farmers doesn't provide a straightforward "pro-growth" solution to the dilemma. In the local context, if the bay becomes too polluted to support crabs and tourists, Maryland's economy as a whole will suffer as much as if there were an agricultural collapse.

For that reason, politicians in Maryland and across the country must start looking at the economics of agriculture and environmentalism through a wider lens. In Maryland and Virginia, legislatures need to consider more stringent measures to control water pollution. Farmers, most of whom receive taxpayer-funded subsidies, should be required to contribute to the public good -- and that includes helping to keep the water clean.