Restoring floodplains and riparian forest: Lessons from the Cosumnes River

Naturally-functioning floodplains provide a variety of ecosystem services, including habitat for diverse species.  California’s rivers are highly altered, with levees that disconnect floodplains, dams that alter flows, and land use practices that have destroyed most than 90  percent of riparian habitat.  Restoration efforts that reinstate natural hydrologic processes are likely to produce more resilient, functioning ecosystems. At the Cosumnes River Preserve, levees were intentionally breached to restore hydrologic connection between the lower Cosumnes River and its floodplain. This river remains unregulated and floods often in winter and spring. Large floods transport sediment and woody debris through breaches to create complex floodplain topography and stimulate establishment of willow and cottonwood riparian forests. Smaller floods transport nutrients, which stimulate pulses in algal and zooplankton productivity, and maintain connectivity between river and floodplain.  Seasonally flooded habitats support up to 33 species of native and alien fish.  Native species, which are better adapted to the seasonal pattern of flooding, utilize the floodplain for spawning (Sacramento splittail, blackfish, hitch) and rearing (chinook salmon, Sacramento sucker, pikeminnow).  Juvenile chinook salmon grew larger in ephemeral floodplain habitats than in the river or permanent ponds.  The Nature Conservancy is using these lessons learned to benefit future restoration at the Preserve and other floodplain rivers in California and beyond.  Opportunities to restore natural hydrologic processes that sustain floodplain and riparian habitats, however, may be limited on many rivers. Challenges include dams, flood management, channel incision, surrounding land uses, and competing goals for species and habitats.