Former Channel Habitats of the Sacramento River: Physical and Ecological Processes and Restoration Potential Presented by Ingrid C. Morken By G. M. Kondolf, J. Constantine, H. Piegay, I. Morken J. Levrat, M. Michalkova Sacramento River Restoration Science Conference April 10, 2007

Former Channel Creation Former channels, including oxbow lakes, are typically abandoned meander bends, cut off from the main channel. Two types of cut off mechanisms occur: Neck cut-off Chute cut-off

Sacramento River Former Channel Habitats: A Typical Evolutionary Pattern 1980 - Main channel occupies meander bend. - Side channel exists. 1999 Downstream connection grows smaller. Riparian forest colonizes open areas adjacent to former channel. 1984 - Chute cut-off occurs. - Upstream plug formed.

Sacramento River Former Channel Habitat 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 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

METHODS FIELD 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 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

EXAMPLE STUDY SITES Little Packer Lake Packer Lake

EXAMPLE STUDY SITES Kopta Slough La Barranca

Hydrologic Connectivity to 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 = 11,300 cfs River discharge = 26,780 cfs

Sediment Depths and Average Annual Sedimentation Rates of Former Channels Sediment depths ranged from 0 to >5 m. Average approx. 2 m deep. Sedimentation rates varied from 0 to 18 cm/year. Average annual sedimentation rate = (cm/year) Average fine sediment depth (cm) Age (years)

Sedimentation Rate Regression Model

Granulometric Data and 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 and Relationship to Diversion Angles

Granulometric Data and Relationship to Diversion Angles

Diversion Angle vs. Infilling Rate

Aerial 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. Straight (isolated) Straight (connected)

DISCUSSION 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 & RESTORATION 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