Lake Erie’s travails — and now, Toledo’s — are but the most visible manifestation of a pollution problem that has grown as easily as it has defied solution. Once the shining success of the environmental movement — Lake Erie was mocked as dead in the 1960s, then revived by clean-water rules — it has sunk into crisis again as urbanization and industrial agriculture have spawned new and potent sources of phosphorus runoff.

In Lake Erie’s case, the phosphorus feeds a poisonous algae whose toxin, called microcystin, causes diarrhea, vomiting and liver-function problems, and readily kills dogs and other small animals that drink contaminated water. Toledo was unlucky: A small bloom of toxic algae happened to form directly over the city’s water-intake pipe in Lake Erie, miles offshore.

Beyond the dangers to people and animals, the algae wreak tens of billions of dollars of damage on commercial fishing and on the recreational and vacation trades. With conservationists and utility officials like Mr. Moline, representatives of those industries have for years called for some way to limit the phosphorus flowing into waterways.

There are practical and political reasons, environmental activists and other say, why it has not happened. The biggest, perhaps, is that the government has few legal options to impose limits — and voluntary limits so far have barely dented the problem.

The federal Clean Water Act is intended to limit pollution from fixed points like industrial outfalls and sewer pipes, but most of the troublesome phosphorus carried into waterways like Lake Erie is spread over thousands of square miles. Addressing so-called nonpoint pollution is mostly left to the states, and in many cases, the states have chosen not to act.

Beyond that, the Supreme Court has questioned the scope of the Clean Water Act in recent years, limiting regulators’ ability to protect wetlands and other watery areas that are not directly connected to streams, or that do not flow year-round.