Floodplain restoration has been advocated as a means to restore several ecological services associated with floodplains: water quality improvement, fish rearing habitat, wildlife habitat, flood control, and groundwater recharge. A history of agricultural encroachment on the lower Cosumnes River has resulted in extensive channelization and levee construction. In fall 1998, an experimental floodplain was established by breaching a levee in order to restore the connection between the main channel and its historic floodplain. In this study, we examined how effective this newly restored floodplain was at processing nitrate (NO3-) before reentering the main channel downstream. Two methods were used to examine nitrate loss. In December 2001, we determined denitrification potentials of the floodplain soils before seasonal flooding inundated the floodplain. Next, we conducted a series of field soil column (mesocosm) experiments from March to June 2002 to study NO3–N loss from the overlying water in both sandy and clayey soils and at three levels of NO3–N (ambient, +1 mg N l(-1), +5 mg N l(-1)). In addition, we examined NO3–N loss from mesocosms with water only to determine if loss was related primarily to soil or water column processes. Denitrification potentials were highly variable ranging from 1.6 to 769 ng N2O-N cm(-3) h(-1). In addition, the denitrification potential was highly correlated with the amount of bioavailable carbon indicating that carbon was a limiting factor for denitrification. Nitrate-N loss rates from mesocosms ranged from 2.9 to 21.0 mu g N l(-1) h(-1) over all treatments and all 3 months examined. Significant loss of NO3–N (60-93%) from the water only treatment only occurred in June when warmer temperatures and solar radiation most likely increased NO3–N uptake by phytoplankton. When scaled up, potential NO3–N loss from the restored floodplain represented 0.6-4.4% of the annual N load from the Lower Cosumnes River during a typical wet year and similar to 24% during a dry year. During dry water years, the effectiveness of the floodplain for reducing nitrate is limited by the amount of N supplied to the floodplain. Results from this study suggest that restored floodplains can be an effective NO3- sink.
In 1997, four breaches were cut into the east and south levee in order to reconnect the riparian floodplain with the adjacent Cosumnes River. Subsequent storms carried large woody debris, sand, water and associated nutrients onto the floodplain. On the lower Cosumnes River, levees are now being intentionally breached in an effort to restore river-floodplain connectivity. These intentional levee breaches allow water from the channel to spill into retired farmland during storm events, thus simulating natural flooding events. The development of floodplain topography from sediment deposition during floods on the lower Cosumnes River has led to the establishment of riparian vegetation and recruitment of large woody debris
|Climate change impacts||Effect of Nbs on CCI||Effect measures|
|Reduced water quality||Positive||Nitrate-N loss rates|
The study site was located within the confines of the Cosumnes River Preserve, a restored flood- plain habitat located 34 km south of Sacramento, CA.