Similarly, a 2006 study by the federally chartered National Research Council found that these coal-burning byproducts “often contain a mixture of metals and other constituents in sufficient quantities that they may pose public health and environmental concerns if improperly managed.” The study said “risks to human health and ecosystems” might occur when these contaminants entered drinking water supplies or surface water bodies.

Image Construction crews worked under lights Tuesday night to clear mud and fly ash from Swan Pond Road and the railroad tracks leading to the T.V.A. power plant in Kingston, Tenn. Credit... J. Miles Carey/Knoxville News Sentinel

In 2000, the Environmental Protection Agency proposed stricter federal controls of coal ash, but backed away in the face of fierce opposition from utilities, the coal industry, and Clinton administration officials. At the time, the Edison Electric Institute, an association of power utilities, estimated that the industry would have to spend up to $5 billion in additional cleanup costs if the substance were declared hazardous. Since then, environmentalists have urged tighter federal standards, and the E.P.A. is reconsidering its decision not to classify the waste as hazardous.

A morning flight over the disaster area showed some cleanup activity along a road and the railroad tracks that take coal to the facility, both heaped in sludge, but no evidence of promised skimmers or barricades on the water to prevent the ash from sliding downstream. The breach occurred when an earthen dike, the only thing separating millions of cubic yards of ash from the river, gave way, releasing a glossy sea of muck, four to six feet thick, dotted with icebergs of ash across the landscape. Where the Clinch River joined the Tennessee, a clear demarcation was visible between the soiled waters of the former and the clear brown broth of the latter.

By afternoon, dump trucks were depositing rock into the river in a race to blockade it before an impending rainstorm washed more ash downstream.

The spill, which released about 300 million gallons of sludge and water, is far larger than the other two similar disasters, said Jeffrey Stant, the director of the Coal Combustion Waste Initiative for the Environmental Integrity Project, an environmental legal group, who has written on the subject for the E.P.A. One spill in 1967 on the Clinch River in Virginia released about 130 million gallons, and the other in 2005 in Northampton County, Pa., released about 100 million gallons into the Delaware River.

The contents of coal ash can vary widely depending on the source, but one study found that the mean concentrations of lead, chromium, nickel and arsenic are three to five times higher in the Appalachian coal that is mined near Kingston than in Rocky Mountain or Northern Plains coal.