River Floodplain Restoration Experiments Offer a Window into the Past

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

“…Following levee‐breach experiments in the late‐1990s, Florsheim and Mount () presented a conceptual model for the generation of floodplain topography at engineered levee breach sites, which has guided expectations of floodplain topography responses to intentional levee breaches elsewhere along the Cosumnes River. It has also highlighted the efficacy of using intentional levee breaches to initiate process‐based restoration of floodplain riparian and wetland ecosystems (Mount et al, ), in concert with, or in lieu of, labor‐intensive horticultural approaches (Reiner, ; Swenson et al, ; Swenson et al, ). However, this existing model of floodplain topography change in response to intentional levee breaching was derived from a largely uniform starting point: narrow levee breaches (<75 m) opening onto graded floodplain areas.…”

“…The lower Cosumnes River (Figure ) has long been used as a landscape‐scale study area to assess floodplain ecosystem responses to intentional breaches in riverside levees (Swenson et al, ). Part of the Cosumnes River Preserve, the study area provides a unique laboratory within which to observe floodplain ecosystem responses following connection of unregulated seasonal flood pulses in the Cosumnes River—the largest undammed river draining the western flank of the Sierra Nevada—to proximal floodplain areas.…”

“…Despite increasing urban pressures on the eco‐agricultural matrix found in the Cosumnes River basin, the lack of flood control beyond riverside levees has resulted in comparatively low human population densities, which in turn has allowed for a unique approach to floodplain management. Throughout the 1990s, levee breaches along the lowest 3.5 km of the Cosumnes River immediately upstream from its confluence with the Mokelumne River were excavated to promote the development of sand splays and the establishment of riparian forests on former tomato fields (Mount et al, ; Swenson et al, ; Swenson et al, ). The depth of channel incision along the lower Cosumnes River has largely precluded the possibility of channel avulsion through the breaches.…”

“…The lateral connectivity of water and sediment during seasonal flood pulses is a key factor controlling the abiotic processes, biotic composition, and ecological dynamics that determine floodplain community structure (Junk, Bayley, & Sparks, ; Tockner, Malard, & Ward, ). However, riverside levees and dykes largely eliminate floodplain hydrologic connection in human‐dominated riverscapes, making the re‐establishment of hydrogeomorphic processes to lowland river floodplains through the breaching of riverside levees an emerging floodplain restoration practice (Swenson, Reiner, Reynolds, & Marty, ; Swenson, Whitener, & Eaton, ).…”

Extreme hydrological events and the ecological restoration of flowing waters

“…Consequently, ‘floodplain activation floods’ may no longer inundate the floodplain. However, occasional large ‘floodplain maintenance floods’ and the rare extreme event of ‘floodplain resetting floods’ may stimulate floodplain restoration, provided that before these floods, there are restorative interventions on the floodplain, such as increasing lateral connectivity (Opperman et al ., ; Swenson et al ., ). Such restorative measures include the removal of levees and restoration of floodplain vegetation as advocated by Hey & Philippi () following the devastating floods along the Mississippi River in 1993.…”