Landscape Response to Disturbance

USGS scientists examining landslides in the Tuolumne watershed, California, caused by an extreme rain event in 2018. Photo credit: Ian Buckley

Climate models project that in the future the western U.S. will experience more extreme rain events, greater wildfire activity, and more pronounced swings between extreme drought and extreme wet conditions. These changes could result in landscapes shedding more sediment from hillslopes and transporting it along rivers to the coast. At the same time, removal of aging dams is becoming common in the U.S.; dam removal (and some other human activities) releases sediment downstream, changing the river and coastal environments. Our research is guided by the questions: How does the scale of landscape response (such as the amount of sediment generated) correspond to the scale of disturbance (the amount of storm rainfall)? What controls lag times in landscape response signals—how long after a disturbance do its effects appear downstream, and how long do they last? How do superimposed disturbances, such as fire and subsequent storms, or a dam removal followed by extreme rain, combine to drive landscape evolution? How will variations in landscape sediment output affect the amount of sediment reaching the coast, and how might that affect the shape and evolution of beaches?
 

In the aftermath of the 2018 Carr Fire, northern California, sediment has eroded from burned hillslopes and accumulated in some areas of Whiskeytown Lake. A recent sediment deposit is shown here in the Whiskey Creek section of the lake. USGS scientists study landscape evolution after fires and storm rainfall to determine how intensively these disturbances impact affected areas and for how long. Photo credit: Amy East, USGS

Currently active tasks:

1. Dam removal. Our team studies river and coastal response to sediment released by large dam removals, in collaboration with other federal, state, tribal, and academic researchers. Research efforts include long-term studies of the Elwha River, Washington (largest dam removal worldwide), the Carmel River (largest dam removal in California), and Klamath estuary (four pending dam removals in California and Oregon).
    
2. Landscape response to extreme rainfall. Extreme rain can cause major landslides and flooding, greatly increasing the amount of sediment moving along rivers to the coast. We study effects of extreme rain in, for example, the San Lorenzo and Tuolumne Rivers (California), collaborating with the Coastal Change Hazards program, USGS Geology, Minerals, Energy, and Geophysics Science Center, and others.
    
3. Post-fire sediment mobilization. Hillslopes commonly shed large quantities of sediment in the aftermath of a wildfire, with potential hazards for downstream communities, infrastructure and water supply. The size and duration of these effects vary widely among landscapes, and have not been measured in detail for many regions. We are collaborating with the National Park Service and other partners to investigate post-fire landscape evolution after the 2018 Carr Fire, northern California, a federally declared disaster. Our efforts focus on Whiskeytown National Recreation Area, evaluating sediment movement that affects Whiskeytown Lake and surrounding watersheds.

Evolution of the shoreline around the Elwha River mouth, Washington, before, during and after dam removal, 2011–2017. Two large dams were removed from the Elwha River between 2011 and 2014 in the largest dam removal worldwide thus far, releasing more than 20 million tons of sediment downstream. These images show the effects of new sediment depositing around the river mouth and being reworked by waves and currents. The Elwha River and other recent dam-removal sites are generating new understanding of ways in which river and coastal systems respond to human activity. Image from Warrick and others, 2019, Scientific Reports.

USGS scientists surveying the Elwha River channel, Washington, to study its response to two large dam removals. Photo credit: J.T. Minear