Not all breaks are equal: Variable hydrologic and geomorphic responses to multiple intentional levee breaches along the lower Cosumnes River, California

Abstract:

Floodplain and riparian areas provide some of the most dynamic and complex habitat mosaics found on Earth. The lateral connectivity of water and sediment during seasonal flood pulses is a key factor controlling the abiotic and biotic processes that determine floodplain habitat structure. However, riverside levees have largely eliminated floodplain hydrologic connection in human-dominated riverscapes, resulting in widespread loss of floodplain habitats and associated ecosystem services. Recently, an emphasis has been placed on the re-establishment of hydrogeomorphic processes to lowland river floodplains through the breaching of riverside levees, with the lower Cosumnes River in California’s Central Valley being one exemplar for this approach to floodplain ecosystem restoration. Observations following more than two decades of intentional and accidental levee breaches identified highly variable hydrologic and geomorphic responses to restoration actions. Principal among these observations is that narrow (<75 m) breaches generate archetypal, lobate crevasse-splay complexes, but result in minimal geomorphic change within the main river channel. However, recent observations from a wide (>400 m) levee breach identified the development of multiple and overlapping crevasse-splay complexes, along with upstream bed erosion and downstream aggradation in the main river channel. We hypothesize that geomorphic change in the main river channel adjacent to this large levee breach is the result of flood basin filling and associated water surface drawdown at flood stage. Additionally, longitudinal variability in channel capacity results in large differences in floodplain hydrologic connectivity thresholds and the duration of floodplain inundation, which ultimately influence biotic responses to flooding. Our observations suggest not all levee breaches are created equal, and an understanding of levee breach and adjacent main channel geometry is key to predicting and evaluating both hydrogeomorphic and biotic responses to this burgeoning approach to floodplain ecosystem restoration.