1. Functional Floodplains on the Central Valley’s Regulated Lowland Rivers: A Vital Link to the Delta Betty Andrews, PE Philip Williams & Associates, Ltd. 9/25/07 CALFED Brown Bag Series

2. Introduction 1.Floodplain inundation functions 2.Anthropogenic alterations 3.What we don’t know about floodplains 4.Management implications 5.What we need to learn for present-day floodplain restoration and management

3. What we know: floodplain functions  Support primary and secondary productivity, with onsite and downstream benefits Jassby and Cloern (2000): Increased inundation of floodplain habitat probably offers the greatest potential for enhancement of high-quality organic matter to the food web of the San Francisco estuary.

4. graphic is a schematic of system function as described by Grosholz & Gallo (2006) What we know: floodplain functions Ahearn et al., 2006: showed the importance of disconnection for delivery of organic material from the floodplain to the river

5. What we know: floodplain functions  Support primary and secondary productivity, with onsite and downstream benefits Grosholz & Gallo (2006): “The floodplain areas had benthic invertebrate biomass and zooplankton biomass that were one to two orders of magnitude greater than in the river channel, which supports similar results from other floodplain systems.” Lehman et al. (2007): The floodplain exported “14–37% of the combined floodplain plus river load of total, diatom and green algal biomass and wide diameter cells to the estuary downstream, even though it had only 3% of the river streamflow.… [T]he quantity and quality of riverine phytoplankton biomass available to the aquatic food web could be enhanced by passing river water through a floodplain during the flood season.”

6.  Provide fish spawning and rearing habitat (e.g., Chinook salmon and splittail) Moyle et al. (2007) “Given the importance of floodplain habitat to splittail and Chinook salmon, and likely importance to other native species, the long-term decline in abundance of native fishes and fisheries may be at least in part related to the loss of floodplain habitat.” What we know: floodplain functions

7.  Support enhanced growth rates in fish (may support adult recruitment) Sommer et al. (2001); Jeffres 2006: Seasonally-inundated floodplains represent one of the most important rearing habitats for young Chinook salmon, likely enhancing their growth rates and resultant survival success. What we know: floodplain functions

8. Photo by Jeff Opperman; from Cosumnes River field study by Carson Jeffres What we know: floodplain functions Fish reared in the river Fish reared on the floodplain

9.  Foster recruitment of riparian vegetation Figure from Stella, 2005; adapted from Mahoney and Rood 1998 Figure from Stella, 2005 What we know: floodplain functions

10.  May affect avian reproductive success (recent evidence suggests reduced nest predation with more spring flooding, providing evidence that floods regulate riparian nest predator populations)  Serve different ecological functions through a variety of landscape elements and vegetative cover What we know: floodplain functions

11.  Through change -- such as high flow disturbance and lateral erosion and attendant new floodplain construction – create ecologically-valuable dynamic habitat mosaics What we know: floodplain functions

12. Source: Jeff Opperman

13. Anthropogenic change  Construction of levees  Channel incision  Change in hydrograph  … and bank hardening, loss of riparian corridor, changes in land use, channelization, etc. Undisturbed channel Loss of connectivity even if levees are removed

14. Concept of nested floodplains

15. Source: Jeff Opperman Small, frequent floods are key for some processes! -- probably those most critical to the Delta ecosystem.

16. Anthropogenic change peak in May or June peak April - July Tuolumne River – timing and quantity change Median peak mean monthly discharge reduced by > 50%

17. Anthropogenic change Tuolumne River: 5-day flood flow change 1897-1922 1970-1993 -- reduction of > 50%

18.  How did historic floodplain inundation patterns vary longitudinally? –Were floodplains subject to multiple extended floods each year limited to the very lowest reaches? How low? –What reach of alluvial lowland river was subject only to annual or semi-annual flooding, perhaps of shorter duration? –How did these two extremes differ in habitat/functions provided? What we don’t know

19.  What are the characteristics of today’s floodplains? –Where are floodplains located that are routinely activated at an ecologically-significant frequency? –How extensive are they? –What is the prevalence of floodplains with different inundation regimes? What we don’t know

20.  What are the characteristics of today’s floodplains? What we don’t know Timing, duration, and frequency:  March 15 - May 15  7-day duration (continuous)  Equaled or exceeded 2 out of 3 years PWA with Opperman (2006) Evaluated relative stage. Looked at 4 reaches, Sacramento River: Vina to Freeport Yolo Bypass Found: stage associated with this discharge was well below adjacent floodplain elevations -- except in the Yolo Bypass.

21. Management implications  Restoration of floodplain functions will require specific attention to how those floodplains are managed, particularly with regard to inundation regime, and especially for small, frequent floods.  Small, frequent flood events upstream will likely provide the most benefit to the Delta.  Significant alterations to river flow regimes, channel-floodplain geometry, or channel hydraulics may be necessary to achieve improved floodplain ecological function.  The opportunity areas for certain kinds of readily-restorable floodplain restoration projects may be quite limited.  The presence of alien species may require alteration or management of some natural floodplain features to maximize benefits.  The hydrologic regime outside of the flood cycle is critical to the success of many flood-reliant species.

22. What we need to learn  What are the relative ecological benefits of different floodplain land uses?  What are the characteristics of the inundation regimes that provide different mixes of ecological functions?  Where are the best opportunities to restore different kinds of ecological functions?  How will present landscape trends affect floodplain restoration opportunities and constraints in the future, including sea level rise?

23. Discussion