By Andrew Nichols
California’s drought plays on, and a recent series of storms to start the New Year have done little to change this broken record. However, promising weather conditions suggest a change of tune may be coming soon. This is exciting news for drought-stricken California, bringing hope of full reservoirs and an extended spring ski season. It has also put those of us who study floodplains on high alert, with every new storm ushering anticipation of rising rivers and possible flooding of riverside ecosystems and flood bypasses (and hopefully not nearby communities!).
For over 20 years, scientists at the Center for Watershed Sciences have studied the complex dynamics of floodplain ecosystems throughout the 50,000-acre Cosumnes River Preserve near Lodi in California’s Central Valley. At selected locations throughout the preserve, The Nature Conservancy has intentionally breached riverside levees, allowing floodwaters to flow over former agricultural lands. Additional levees away from the river allow flooding in a largely controlled experimental setting, while also minimizing flood risks to surrounding fields and communities.
Many forget that the Central Valley was once a vast flood basin, with myriad river channels, wetlands, lakes and forests forming a complex and dynamic floodplain ecosystem dependent on seasonal flooding . The removal of levees is an attempt to re-introduce floodwaters and kickstart the restoration of this floodplain ecosystem. This is “process-based” restoration at its finest – where the river, not people, does most of the restoration “work.” The results of early levee breach experiments along the Cosumnes River in the late 1990s were remarkable.
The reintroduction of floodwaters dramatically altered the former agricultural landscape. Sediment-laden floodwaters deposited sand on the newly accessible floodplain, altering the once level ground and promoting the establishment young cottonwoods and willows. Today, a dense forest occupies what was once a tomato field. Floodwaters also created an ideal nursery for growing young salmon, while simultaneously refilling local aquifers. This became a model for the process-based restoration of floodplain areas throughout the Cosumnes River Preserve and elsewhere in California.
In the fall of 2014 – in the midst of California’s extended drought – The Nature Conservancy breached several more levees along the Cosumnes River. And once again scientists from UC Davis and UC Merced are on the scene to monitor the ecosystem response. A single storm in February 2015 produced some tantalizing initial floodplain ecosystem responses to a very brief (3 days) period of flooding. As expected, floodwaters brought water, sediment and seeds onto the floodplain, initiating the natural recruitment and growth of young cottonwoods and willows. Flooding also added water to local groundwater aquifers. However, this brief spate of flooding also brought an unexpected change to the Cosumnes River.
Upstream from one of the new levee breaches, the Cosumnes River dramatically lowered its bed. In some places, floodwaters cut through more than 2 meters of a hard soil layer that previously resisted erosion by the river. Such changes to the Cosumnes River channel are not found near any other levee breach. We hypothesize that a novel levee breach design, where more than 250 meters of old levee were removed, created unique hydraulic conditions that allowed water in the river upstream from the breach to rapidly accelerate and erode the channel bed. Observations such as this facilitate understanding of unique geomorphic responses to different levee breach designs, and enable more effective predictions of channel responses to levee breaches elsewhere.
UC Davis and UC Merced scientists are hopeful that strengthening El Niño conditions and changing weather patterns in the Eastern Pacific Ocean will lead to prolonged periods of flooding along the Cosumnes River during the winter and spring of 2016. The experimental floodplain restoration sites along the lower Cosumnes River are proving grounds for passive and active floodplain restoration techniques, where “lessons learned” can be exported to other restoration sites throughout California.
While many people in California dread the prospect of prolonged rains and potential flooding, those of us trying to study floodplain ecosystems in the midst of a drought cannot wait for the next opportunity to put on our waders and watch the floodplain come alive under our feet.
Andrew Nichols is a geomorphologist at the Center for Watershed Sciences.
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