The river outbreak has produced few consequences beyond canceled boat races and warnings in five states to avoid contact with the river. But scientists say it is not to be taken lightly. The toxin, microcystin, causes diarrhea, vomiting and liver damage, and it has been known to kill animals unlucky enough to drink water tainted with it. Last year’s Lake Erie bloom, an annual event, peaked over the municipal water intake for Toledo, forcing the city to shut down drinking water supplies to 400,000 residents for four days in August.

What causes the blooms is clear: Tides of phosphates and nitrates, flushed into the river from fertilized fields, cattle feedlots and leaky sewers, provide food for the algae, which are actually bacteria. Hard rains in parts of the Ohio Valley basin this summer appear to have washed more than the usual amount of the chemicals into the river and into Lake Erie.

The algae need hot weather, sun and still water to flourish. All were abundant in August and September, when a dry spell reduced the Ohio’s flow and cleared its usually muddy waters.

But if this year’s algae epidemic can be explained, why it has not appeared before is a murkier question.

The river absorbs a colossal amount of phosphorus — nearly 42,000 tons in an average year — but “while we have seen a very slight increase in phosphorus over the years, it’s not something that stands out,” Richard Harrison, the executive director of the Ohio River Valley Water Sanitation Commission, said in an interview. “It’s still something we’re investigating.”