Functional Floodplainson the Central Valley’s Regulated

Lowland Rivers:A Vital Link to the Delta

Betty Andrews, PEPhilip Williams & Associates, Ltd.

9/25/07 CALFED Brown Bag Series

Introduction

1. Floodplain inundation functions2. Anthropogenic alterations3. What we don’t know about floodplains4. Management implications5. What we need to learn for present-day floodplain

restoration and management

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.

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

Time

water stage

zooplankton (phytoplankton: "similar pattern")

temperature

connection

disconnection

peak reached in 2-3 weeks

connectivity stage

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.”

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

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

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

Foster recruitment of riparian vegetation

Figure from Stella, 2005; adapted from Mahoney and Rood 1998Figure from Stella, 2005

What we know: floodplain functions

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

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

Source: Jeff Opperman

Anthropogenic change

Construction of levees Channel incision Change in hydrograph … and bank hardening,

loss of riparian corridor, changes in land use, channelization, etc.

Undisturbed channelLoss of connectivity even if levees are removed

Concept of nested floodplains

Source: Jeff Opperman

Small, frequent floods are key for some processes!

-- probably those most critical to the Delta ecosystem.

Anthropogenic change

peak in May or June

peak April - July

Tuolumne River – timing and quantity change

Median peak mean monthly discharge reduced by > 50%

Anthropogenic change

Tuolumne River: 5-day flood flow change

1897-1922

1970-1993-- reduction of > 50%

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

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

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.

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.

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?

Discussion