Former Channel Habitats of Former Channel Habitats of the Sacramento River:the Sacramento River:

Physical and Ecological Processes Physical and Ecological Processes and Restoration Potentialand Restoration Potential

Presented by Ingrid C. MorkenPresented by Ingrid C. Morken

By G. M. Kondolf, J. Constantine, H. Piegay, I. MorkenBy G. M. Kondolf, J. Constantine, H. Piegay, I. MorkenJ. Levrat, M. MichalkovaJ. Levrat, M. Michalkova

Sacramento River Restoration Science ConferenceSacramento River Restoration Science Conference

April 10, 2007April 10, 2007

Former Channel Former Channel CreationCreation

Former channels, including oxbow lakes, are typically abandoned meander bends, cut off from the main channel.

Two types of cut off mechanisms occur:

1) Neck cut-off2) Chute cut-off

Sacramento River Former Channel Habitats:Sacramento River Former Channel Habitats:A Typical Evolutionary Pattern A Typical Evolutionary Pattern

1980

- Main channel occupies meander bend.

- Side channel exists.

1984

- Chute cut-off occurs.

- Upstream plug formed.

1999

- Downstream connection grows smaller.

- Riparian forest colonizes open areas adjacent to former channel.

Sacramento River Former Channel Sacramento River Former Channel Habitat ValueHabitat Value

• Provide important habitat different from the main channel, supporting unique vegetation assemblages and wildlife species

• Evolutionary stages provide different types of habitat• Important areas for nascent and existing riparian forests

STUDY OBJECTIVESSTUDY OBJECTIVES Document and assess former channel evolution processes and patterns

- Field data collection- Aerial photograph analysis

Identify restoration potential and management implications of former channel evolution trends

METHODSMETHODSFIELD DATA COLLECTION• Measured water depths, water surface elevations, and

surficial velocity• Measured fine sediment depths• Assessed granulometric data from sediment cores

AERIAL PHOTOGRAPH ANALYSIS• Measured diversion angle between main and former

channels• Delineated, measured various dimensions of former channel

sites with time series of aerial photographs

STUDY SITESSTUDY SITES

• 28 former channel sample sites located between River Miles 161.2 and 236.8

• Former channel sites varied by the following:- Age- Hydrologic connectivity to main channel- Degree of terrestrialization

Packer Lake

Little Packer Lake

EXAMPLE STUDY SITESEXAMPLE STUDY SITES

Kopta Slough La Barranca

EXAMPLE STUDY SITESEXAMPLE STUDY SITES

Hydrologic Connectivity Hydrologic Connectivity to Main Channelto Main Channel

• Water Depths

Ranged from consistently shallow (< 1 m) systems to 3-4 m deep.

• Water Surface Elevations

Some former channels more responsive than others, reflecting variations in hydrologic connectivity to main channel.

• Surficial Velocity

Velocity responses varied at different river discharges. Greater changes in velocity reflect greater hydrologic connectivity to main channel.

River discharge = 26,780 cfs River discharge = 11,300 cfs

Sediment Depths and Average Annual Sedimentation Sediment Depths and Average Annual Sedimentation Rates of Former ChannelsRates of Former Channels

0

1

2

3

4

5

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910

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20

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150Former Channel Age (years)

Ave

rag

e A

nn

ual

Sed

imen

tati

on

Rat

e (c

m/y

ear) Average annual

sedimentation rate = (cm/year)

Average fine sediment depth (cm)

Age (years)

• Sediment depths ranged from 0 to >5 m. Average approx. 2 m deep.

• Sedimentation rates varied from 0 to 18 cm/year.

Sedimentation Rate Regression ModelSedimentation Rate Regression Model

R² = 0.37

Granulometric Data Granulometric Data and Relationship to Diversion Anglesand Relationship to Diversion Angles

• Former channels with varying diversion angles reveal different granulometric profiles with depth.

- Greater than 75° dominated by silt and clay above gravel bases.

- Less than 50° shows occurrence of mostly sand to silt• Diversion angle can control formation of sediment plug

Granulometric Data Granulometric Data and Relationship to Diversion Anglesand Relationship to Diversion Angles

Granulometric Data Granulometric Data and Relationship to Diversion Anglesand Relationship to Diversion Angles

Diversion Angle vs. Infilling RateDiversion Angle vs. Infilling Rate

Aerial Photograph AnalysisAerial Photograph Analysis• Delineated, measured length, width, water surface area, vegetated surface area of sites on 1942, 1962, 1985, and 1999 aerial photographs

• Categorized former channels according to size and connection to main channel. Preliminary results suggest that less sinuous former channels with a strong downstream connection tend to have lower sedimentation rates.

0

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Sed

ime

nta

tion

Rat

e (

cm/y

r)

0 20 40 60 80 100 120 140yr since cut-off

Straight (active)

Straight (backwaters)

Meander

Straight (isolated)

Straight (connected)

DISCUSSIONDISCUSSION

• Sedimentation rates tend to even out over time, with slower filling former channels catching up to faster filling former channels

• A strong relationship occurs between diversion angle and the evolutionary pattern of former channels

• Former channel geometric form did not appear to have a strong relationship with sedimentation rates

• Former channels show a wide range of hydrologic connectivity to main channel, reflecting evolutionary stage and ecological function

MANAGEMENT IMPLICATIONS & MANAGEMENT IMPLICATIONS & RESTORATION POTENTIALRESTORATION POTENTIAL

• Long-term management and restoration strategies must consider evolutionary trends

• Restoring processes which promote former channel formation are essential. E.g. Riprapped banks inhibit lateral channel migration

• Potential restoration sites for riparian forests