Former agricultural fields restored to riparian floodplains along the lower Tuolumne River, CA: Inundation patterns, fish use, and revegetation results

Agricultural encroachment and flow regulation have impaired natural floodplain functions of many Central Valley rivers, resulting in significant loss of native riparian forest and impacts to native fish and wildlife. In 2004, restoration of 238 acres of former agricultural fields along the lower Tuolumne River was initiated by notching surrounding berms to restore hydrologic connectivity, and planting 60 acres with native riparian trees and shrubs. Post-implementation monitoring began in 2005 to evaluate floodplain hydrology and sedimentation, fish use, survival and growth of planted vegetation, and natural recruitment of native woody vegetation.  Floodplain inundation began at flows of 4,000 cfs (a 2-year recurrence interval event), and suitable depth for fish was achieved at approximately 6,000 cfs. Juvenile Chinook salmon were observed only in floodplain areas with measurable water velocity. Prolonged inundation events in 2005 and 2006 resulted in mortality of 64–100% of planted California blackberry, California rose, and box elder on a lower elevation field, while all other planted species had ≥ 70% survival. Fremont cottonwood and willow seed rain occurred within unmaintained experimental areas, but seedling recruitment was limited to one experimental area in 2006. Seedlings survived through the summer growing season despite heavy cover of non-native herbaceous vegetation. Results imply that restoration of floodplain hydrogeomorphic function at this site is modest under regulated flow conditions without lowering floodplain surfaces, although it does provide periodic benefits to aquatic ecosystems during wet years and will continue to improve riparian conditions for terrestrial species as planted and naturally recruited vegetation matures.