By Ryan Peek
Thanks to its Mediterranean climate, California swings from one extreme to another — severe drought, raging wildfires, big floods. These forces often interact and amplify, as we saw all too well this past summer in the scorching of hundreds of thousands of extremely dry forested acres, with the loss of homes and lives. Big floods could be just around the corner, if predictions of a “Godzilla El Niño” hold true.
Researchers with the UC Davis Center for Watershed Sciences watch these dramas play out in several rivers of the Sierra Nevada. As part of a long-term river monitoring project, we track changes in streamflow, temperature and structure of river channels. We also regularly collect data on fish diversity, native frog breeding and the diversity and abundance of aquatic invertebrate.
The data are useful for linking physical and ecological processes, but they don’t always tell the whole story.
A few years ago we began to get a fuller picture after installing a number of small time-lapse cameras at key locations within our monitoring sites. Typically used by hunters for game surveillance, these rugged weatherproof devices are programmed to take a picture every hour.
We compile the photos into time-lapse videos and can overlay river data collected at the same location.
Hydrograph curves showing changes in streamflow take on new meaning when matched with corresponding videos, as shown below. We can see what a daily change of, say, 600 cubic feet per second actually looks like in a river.
Note during winter storms how much muddier the river turns in 2015, compared with 2012. The 2013 Rim fire accounts for the difference. This was the largest Sierra fire in more than a century of recordkeeping. It burned 257,314 acres (400 square miles), much of it within the Tuolumne River watershed. With rain and higher flows, sediment from hill slopes can be mobilized and flushed into the river. The loss of trees from the fire amplified this effect, creating less stable soil in the steep river canyon.
Hydrographs and videos of Tuolumne-Clavey confluence
1. January – July 2012
2. January – July 2015
Documenting changes in river systems and comparing them over time is key to understanding the ecological impacts of fires, floods and droughts.
Fire and floods beget erosion and mudslides, clouding waterways with high loads of sediment. High stream flows help “reset” river ecosystems by scouring vegetation, moving sediment and forming new habitat.
Many native California organisms depend on such diverse and dynamic river processes. Dynamism promotes biodiversity.
Videos of the Tuolumne-Clavey confluence are particularly instructive. They capture events before and after the Rim fire. The blaze felled the tree that held our camera, but the device remained attached to the trunk and continued to take pictures and ground level, as shown in this video narrated by Watershed Sciences researcher Eric Holmes:
The drought has minimized the post-fire erosion in the Tuolumne River canyon. Still, with only moderate rain, you see significant increases in turbidity and sedimentation. Visiting the monitoring site this past May, we found a foot or more of silt along the banks.
Heavy sedimentation can have major impacts on native aquatic species such as the foothill yellow-legged frog. The frogs attach their egg masses to rocky substrates in sheltered slow-moving waters of rivers and creeks. The females lay only a single egg cluster a year, so the loss of these breeding grounds to heavy sedimentation can significantly diminish reproductive success.
Similarly, the eggs of salmon and trout require constant flow through their gravel nests to supply enough oxygen for their development. Sedimentation can also smother bottom-dwelling macroinvertebrates, an important food source for many species.
At the same time, the buildup of sediment can provide new areas for riparian vegetation to take hold.
Ironically, the banks of the Tuolumne and Clavey rivers are noticeably lusher these days because of the Rim fire and drought. The post-fire sedimentation and lack of high flows to flush the drought-stricken rivers have allowed riparian vegetation to gain traction.
Time-lapse videos help us understand the ebb and flow of rivers we study. They make it apparent how dynamic these systems can be. They also show the power and importance of fires and floods in creating, maintaining and changing habitat for fish, amphibians and other aquatic life.
Ryan Peek is a doctoral student in ecology at UC Davis.
Animated map of Rim fire (Video, ~1 min.). Open Data City. September 2013
Santos N. 2014. “Remote Sensing, Event-Based Monitoring and Change Detection Using Off-the-Shelf Hardware” (Video, ~13 mins). UC Davis Center for Watershed Sciences. April 30, 2014
Time-lapse videos of Sierra Nevada rivers (Videos). UC Davis Center for Watershed Sciences
Data on stream temperatures and stages. UC Davis Center for Watershed Sciences
Boxall B. “Rim fire’s effects likely to last for decades to come“. Los Angeles Times. Sept. 23, 2013
Jeffres C, Peek R, Ogaz M. “Journey to the bottom of the Rim fire“. California WaterBlog. Sept. 26, 2013
Park H, Cave D, Andrews W, Canepari Z. “After years of drought, wildfires rage in California“. New York Times. July 15, 2015
Swain D. “California fire season explodes“. California Weather Blog. Aug. 4, 2015
Viers J and Santos N. 2014. “Hydrolapse Videography: A Coupled Hydroinformatic Stack for Improved Visual Assessment of River Dynamics“. 11th International Conference on Hydroinformatics. HIC 2014. New York City.