Mt. Rainier's retreating glaciers are making a mess

"Even without climate change, you've got to say: 'Whoa, something is going on here,'" Abbe says.

Debris flows can carry boulders the size of buses and sweep staggering amounts of gravel and sediment into rivers. The bed of the Nisqually River below its namesake glacier has risen by 38 feet since 1910, largely as a result of debris flows from the margins of the rapidly retreating ice, Kennard says.

The park visitor center at Longmire, with its stone buildings and National Park Inn, now sits more than 30 feet below the Nisqually River. The park constructed concrete-reinforced berms to keep the water at bay.

Every river bed in the park is rising, or aggrading, because of the influx of gravel, Kennard says. The rate of buildup has increased nearly 10-fold over the past decade.

The result is a constant and costly battle to keep popular recreation areas throughout the park open. It's a battle that's being lost in many places, like the Westside and Carbon River roads, which are partially closed.

Like conveyor belts, the rivers move the gravel downstream toward more heavily populated areas. A surprise flood that hit the city of Pacific last January can at least partly be blamed on volume reduction in the White River caused by accumulation of sediment, US Geological Survey hydrologist Chris Magirl says.

Magirl, who has examined aggradation rates and historical records for downstream river stretches, sees similar buildup in several locations. But channels appear to be deepening in other places, including portions of the Puyallup and Cowlitz rivers. That type of variation is expected in such a complex system, Mr. Magirl says. But the long-term outlook for the rivers is not good.

"The potential for glacial retreat to add new sediment is historically unprecedented," he says. "Clearly, water and rock are going to flow downhill."

Glacial retreat may be aggravating the flow of sediment, but the basic process is as old as the volcano itself. Past eruptions have unleashed mud flows that smothered surrounding valleys and reached all the way to Puget Sound.

From the 1930s through the 1980s, Pierce County dredged gravel from the Puyallup River system almost every year to reduce the risk of floods, says Lorin Reinelt, program manager for the county's flood-management plan.

Most dredging ended by the early 1990s, as concern for fish habitat took precedence. Officials also realized that digging out gravel provides only a brief fix, at best, Mr. Reinelt says. "In many cases it just fills back up during the next event."

Communities now are trying to figure out what rising levels of gravel and sediment from Mt. Rainier will mean for future flood risks — and what they can do about it.

Short of relocating Longmire, dredging is the only obvious way to keep the river from swallowing the park complex, Kennard says. Downstream, Reinelt says, a more effective approach might be to move levees back to give the rivers more room to spill their banks, meander and deposit gravel without impacting homes or businesses.

"This is a pretty significant issue," he says. "It seems like we're on a trajectory that's not likely to reverse any time soon."

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