The Kachituli Oxbow Restoration Mitigation Site, 20 Years Later: A CRAM-based Assessment

Katchituli grasslands. 

Katchituli grasslands. 

Winter 2014 Ecesis, Volume 24, Issue 4

A successful restoration project can be determined by its successful functionality. These challenges are significant for large-scale projects due to their broad goals and limited ongoing funding. There have been large investments and minimal management along the Sacramento River in northern California. The goal of The Kachituli Oxbow Mitigation Site was to transform a degraded environmental system and replace it with a functioning riparian habitat without hydrologic assistance from the adjacent Sacramento River. The site is like no other due to the hydrology and the water source that is primarily fed from the water table and precipitation. The design of the Kachituli site was portrayed as a radical concept and was expressed with skepticism by many. A CRAM-based assessment was conducted in the spring of 2014 to evaluate the structure and function of the Kachituli system twenty years after its restoration.

Construction of the Kachituli Restoration Site began in 1991, and revegetation of the site began in the winter of 1992, continuing through the following spring. At the time of development, Kachituli was the largest riparian restoration located west of the Mississippi River and the first of its kind in California — a constructed oxbow and riparian habitat. Its initial purpose was to provide native riparian vegetation, which includes cottonwood (Populus sect. Aigeiros) forest, elderberry (Sambucus) savannah, and valley oak (Quercus lobata) woodland. The revegetation design mitigated the loss of comparable habitat at the Lighthouse Marina Project Site, seven miles downriver in West Sacramento. Dr. David Kelley & Associates and Sierra View Landscaping were the chief developers of the Kachituli site. Kelley stated that finding the water table was a primary goal, which would determine the depth of the wetland itself, thus determining the soil and vegetation elevation around it. After excavation of the oxbow had concluded, a drip system was initially installed to sustain the revegetation process; it was to be removed after the revegetation was established during the first 5 years. Kelley’s team managed Kachituli during those years, which meant overseeing irrigation, weed control, and invasive control. Kachituli would continue to function from the natural groundwater.

Kelley and his team conducted intensive background work that included aerial reconnaissance, examination of maps and atlases, and speaking with local experts. The reference sites studied for vegetation composition, soil characteristics, and hydrological components were comparable examples of oxbow lakes —remnants of abandoned channels of the old Sacramento River. Two of these reference sites are Mary Lake and Old River, near Knights Landing. These oxbows sustain dense riparian vegetation and wildlife habitat, such as birds, deer, and raccoons which mirrored what Kachituli was designed to sustain.

The hydrologic function of Kachituli is limited due to disconnect to the Sacramento River’s hydrological sources; instead it functions from groundwater. This functionality was assessed throughout the site and excavation specifications were developed accordingly. To ensure success of a functioning plant community, an irrigation system was put in place, which operated with new pumps and existing pumps that were used for prior agricultural farming.

The design of Kachituli implemented various plant communities into the site. The wetland area was designed to be a suitable habitat for hardstem bulrush and cattail. Willows were planted on the lowest portion of the slope with the ascending portion of the slope suitable for boxelder and cottonwood. The willows planted included four different species, red willow (Salix laevigata), sandbar willow (S. hindsiana), Goodding’s willow (S. gooddingii), and Arroyo willow (S. lasiolepis).

The methodology for vegetation analysis was conducted using standard field techniques through the CRAM framework. Dr. Michelle Stevens, an environmental studies professor at California State University Sacramento, guided the CRAM assessment using a stratified random sample area — the wetland was separated into five transects, where the line intercept method was used to measure densely dominated vegetation.

The study of the Kachituli Mitigation Restoration Site was conducted to compare the effectiveness of a depressional wetland with other depressional wetlands throughout the Bay Delta region. The results of our field research show just how effective a well-designed and constructed wetland habitat can parallel a naturally-occurring wetland habitat.

Kachituli requires an above-average buffer to sustain a functioning and a growing wetland. Compared to other depressional wetlands in the Bay Delta region, Kachituli’s score of 56% was about average, which still speaks well for a constructed wetland.

Kachituli’s hydrological functionality is exceptional. It ranks with some of the most natural and functioning wetland habitats. Some of the 100% hydrologic scores came from wetlands on the California coast, such as Bodega Dunes and Drakes Estero Pond, which are constantly flooded due to high tide. This proves that Kachituli’s hydrology is working.

The physical structure attribute score tells how diverse the patch richness is onsite and the topographical complexity determines the interspersion of patch richness. The score — 75% — was well above average when compared to other depressional wetlands in the region. Again, this provides evidence that, like the natural depressional wetlands, Kachituli is providing habitat for many different species of plants and animals. The plant palette shows how diverse Kachituli has become over the past twenty years — the natural dispersal of vegetation has resulted in a self-sustainable riparian habitat — and Kachituli earned a biotic structure score of 66%, well above average.

After the first 5 monitoring years had ended, the systematic irrigation was halted and the site was to be naturally irrigated by groundwater and precipitation. Kachituli also had minimal management after the monitoring period which brought invasive species, mainly grasses, into the site. Management is certainly something that Kachituli can benefit from, such as weeding, trash clean up, and irrigation hose removal.

Kachituli’s overall CRAM score is a 74%, which is well above average. This percentage comes from the combination of four attribute scores, buffer and landscape, hydrology, physical structure, and biotic structure. Compared to other natural wetlands in the region, Kachituli is seen as an efficient depressional wetland that continues to provide habitat for a diverse array of species.

Kachituli was the first of its kind to provide a constructed riparian habitat that would eventually be sa elf-sustaining and productive ecosystem. This study delivers evidence that the design implemented over 20 years ago is not only working, but continues to produce pristine habitat, which in turn, continues to build a thriving ecosystem for years to come. We can also say the Kachituli wetland is functioning well enough to be used as a reference site for future depressional wetland restoration projects as well as a restoration trajectory due to its favorable comparisons with some of the most functioning depressional wetlands in the Bay Delta region. — by Andrew F. McGuirk, Restoration Resources

References
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