The study uncovered "very, very permeable gravel deposits beneath the mill tailings, beneath the river and beneath the entire Matheson Wetlands Preserve," Solomon added.

Permeable gravel is important for two reasons, he said:

• The layer amounts to a hydrological "superhighway." It's a pathway along which groundwater can migrate under the river. If enough pumping takes place on the tailings side, water might flow toward it. If the reverse is true, water could flow from the tailings toward Moab.

In that case, some controls may be possible through pumping.

• The "very coarse gravels" show that the Colorado River has flooded in the past, bringing in gravel and boulders. "The river has migrated laterally over very large distances through geologic time," he said.

Above the gravel is about 15 feet of fine, silty material. The tailings are on top of the silt, approaching within 1,000 feet of the river in one place.

The tailings amount to "a house that's literally built on sands and silts," Solomon said. "They're not founded on any really competent material," meaning they could easily wash away.

At 24 and 30 feet below the surface, geologists uncovered organic material. A lab in Florida checked the age of the wood and peat through carbon dating.

Radiocarbon dates for samples from the two floods ranged between 1860 and 1980 for the most recent, and between 990 and 1090 AD for the earlier material. That is about when flood waters carried them in.

The samples were taken from a bore hole on the Moab side of the river. But the debris indicate the river's violence throughout the zone.

The report by Gardner and Solomon explains, "The radiocarbon ages of these two samples indicate that there have been two flood events in the last 1,000 years that have scoured down to 24 and 30 feet below present land surface, respectively, at a distance of more than 260 feet from the present river channel."

During January's flooding, the Santa Clara River in southwestern Utah rapidly ate away at its banks.

"The biggest meander was 700 feet," said Jan Sandberg, engineer for the city of St. George. "There was a lot of meandering in lots of areas."

"In the city of Santa Clara, it took a huge bite, took out a bunch of prime real estate," said Dean Cox, emergency services director for Washington County.

An erosion of 700 feet would not quite bring the Colorado River onto the Moab tailings. But the Santa Clara destruction happened with just one flood, and that river is far smaller than the Colorado.

If a big enough flood were to race through the Colorado, said Dianne Nielson, executive director of the Utah Department of Environmental Quality, "We're going to have uranium mill tailings strewn along the banks and sandbars along that river for distances downstream."

That, Nielson added, is unacceptable.

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E-mail: bau@desnews.com