Know Your Watershed: How Flooding and Landslides Affect Streamflow

Streamflow is the volume of water flowing past a certain point on a waterway at any given time of day. Heavy rains and snowmelt can produce runoff at a rate that exceeds the drainage capacity of streams, which results in flooding. Landslides occur as mass movements of soil and rock that are saturated by excess runoff.

For example, when air temperatures are high and snowpack melts too quickly, flooding and landslides may create life-threatening situations. These kinds of events not only carry large amounts of water, they can also carry sediment, silt, and rockfall that can alter the natural channel of a river and destabilize the landscape in rock slides and debris flows.Summer monsoons have the same effect on rivers and streams when a lot of rain falls in a short period of time.

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Sediment flow into the Uncompahgre River between Ridgway and Ouray during an unusually wet September 2022 created undercuts in riverbanks where tree roots now hang exposed.

Landslides (Debris Flow) and Their Effect on Streamflow

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.

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.

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

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

Peak Discharge from High Water Events

USGS stream gages located throughout the Uncompahgre River watershed measure the velocity and depth of water in cubic feet per second. The following stream gage demonstrates a flood event on July 27, 2010. Note the peak discharge of 828 cfs that occurred at 6:15 p.m.

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Please remember to enter the Uncompahgre River Classic peak flow prediction contest by May 1 (for free)!