Mt. St. Helens debris flow, 1991.

Debris Flow and Cowlitz River, Mt. St. Helens Volcanic National Monument, Washington, 2004.

As glaciers retreat, they deposit massive amounts of rocky debris into river systems. This debris fills up river channels when the rivers can’t support the amount of sediment flowing in the water, a process known as aggradation. The river water is forced to change direction and create new channels. This is why Mount Rainier’s rivers have wide, rocky, braided riverbeds. Climate change is accelerating glacial recession and thus increasing the sediment available to rivers to transport downstream. This can lead to new river channels in adjacent floodplains that are in the path of buildings and roads. In addition to melting glaciers, climate change is increasing rain and flooding events. Warmer temperatures means that precipitation falls as rain instead of snow. This can result in rapid aggradation and debris flows.

Hotter weather in the summer can also cause melting glacier water to build up quickly, and if it gets trapped in the ice, it could release suddenly in the form of outburst flooding, another cause of debris flows. Glacial outburst floods are the primary cause of glacially-originated debris flows in the park. Mount Rainier scientists are studying glaciers and river systems to better understand these dynamic forces and have developed a debris flow hazard system to determine conditions when debris flows may occur.

Remember, ANY river in the park is at risk of a debris flow. Rising water level, shaking ground and a rumbling noise may signal a debris flow or lahar. If you are near a river and notice a rapid rise in water level, feel a prolonged shaking of the ground, and hear a roaring sound coming from up valley (often described as the sound made by a fast–moving freight train), then move quickly to higher ground! A location 160 feet (50 m) or more above river level should be safe. Learn more about geohazards.

San Bernardino County has issued evacuation warnings in the Apple and El Dorado burn scar areas in anticipation of a major storm event. 

Sabo dams Mountainous areas of the world have major threats towering over them; sometimes during heavy rains the mountains will come down. A debris flow is a fast-flowing mixture of rock, water, mud, and sand that flows rapidly downhill under the force of gravity. Debris flows can engulf villages and cities rapidly, sometimes with little to no warning. The energy of the flowing water is dangerous enough, but the large chunks of rock can be particularly hazardous as they’ll plow right through a person, car, or house. Sabo dams (also called check dams) rely on technology that has been in use around the world for thousands of years. 

Colombian slide disaster: Here's how the 'debris flow' likely occurred

A lethal mix of gravity, water and dirt combined to create an “avalanche” that tore through a small Colombian city, killing more than 200.

What happened in Mocoa is known as a “debris flow,” which came during an unusually wet season, topped off with a shorter period of intense rain, federal landslide scientist Jonathan Godt said. He said he reviewed images and video of the Mocoa disaster, noting the mountains surrounding the city.

“That very heavy rainfall makes the soil like goo, makes it easy to flow. It just starts sliding down the hillside,” said Godt, coordinator of the U.S. Geological Survey’s landslide hazards program. “My guess is that it was moving very fast and would have been full of rock and boulders and pieces of buildings. For someone experiencing it, an avalanche would be a very accurate description.”