historic flood in Ouray, photo courtesy of Denver Public Library

KNOW YOUR STREAMFLOW: The effect of landslides

Surface water erosion is the primary cause of landslides. When intense periods of rainfall or snowmelt saturate the landscape, it softens the soils mixed within various sizes of rocks and boulders below ground level. The water flow also finds its way into air pockets underground. As the ground becomes more unstable, gravity nudges the heavy soils and rock to slide downhill. When the debris is carried into a river, it can divert water from its natural channel. This is a natural occurrence in mountainous landscapes and part of how landforms evolve over time.

Photo at top: Ouray Flood and Debris Flow on Main Street (August 1909). Source: Denver Public Library Special Collections, X-12700

Landslides that flow from steep canyons leave their marks on a river or stream by altering streamflow. Debris from landslides can narrow or widen channels, create diversions that can undercut and erode river banks, and block agricultural headgates. Sediment also creates “turbidity”–murky water carrying sediment–that can affect water quality for fish and other aquatic life. The sediment itself can cover streambeds and ruin spawning areas as well as smother invertebrates living in the streambed.

Below, the Uncompahgre River filled with orange sediment from a late summer rainstorm that caused a flash flood in a sandstone canyon upstream of Ridgway.

sediment in Uncompahgre River jus south and upstream of Ridgway Reservoir after September 2022 storm