former channel habitats of the sacramento river: physical and ecological processes and restoration...
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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
6
7
8
910
11
12
13
14
15
16
17
18
19
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
2
4
6
8
10
12
14
16
18
20
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
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