Before/After at the Oken Property

Welcome to Before/After, a series where we showcase the long-lasting impact of our restoration work.

In 2017, the Sonoma County Agricultural Preservation and Open Space District (Ag + Open Space) proposed a riparian restoration project on its 76-acre Oken property. The property is located on Petaluma Hill Road, along the northeastern edge of Rohnert Park in Sonoma County. The grassland supports grazing and acts as a bowl for several drainages running down from the surrounding hills. Erosion wasn’t historically a problem, but housing development, the routing of a small stream through roadway culverts and open grazing of the tributaries in recent years had combined to channelize the waterways that cut across the property, creating several steep head cuts.

Before, with active headcuts and erosion

PCI’s biology, design, engineering and planning teams worked together to prepare a plan that would restore the waterways. Their collective vision involved bioengineered repairs, rock grade control, an arch culvert livestock crossing, wildlife firefly fencing and an abundance of willows.

Before urbanization, intermittent tributaries like the ones at Oken would have formed a series of distributary channels that would fan out as they exited the foothills, storing sediment and recharging groundwater. Despite the erosion, an alluvial fan still existed on the property, but it was incised with particularly prominent headcuts. Designers envisioned a willow “sausal” transition for this area (sausal means willow in Spanish, and refers to a thick willow grove). The eroding bank would be re-contoured with soil fill and the disturbed area would be covered with coir blanket and coir mat over native grass seed. A willow thicket would stabilize the slope and allow flows to continue fanning out through use of a bioengineering technique using living willow brush sills and wattles before draining into the main channel. Willow poles would be planted to create the thicket above the sills. At the toe of the slope, along the main channel, two brush sills would be placed with willow wattle anchors.

Willow sausal after construction

PCI’s design plans also called for the construction of three rock stabilization structures. A drop inlet at the downstream end of the swale in the southwestern corner of the property would be protected with a rock basin. Headcuts in the southern end of the property and in main channel toward the center of the property would be repaired. An incised channel segment upstream in the main drainage would be restored with soil fill to create a stable channel bed.

Willow sausal in May 2022

As part of the design process, PCI looked at potential opportunities for additional habitat enhancement, and provided recommendations for wildlife-friendly grazing infrastructure and management of invasive plant infestations.

Construction was completed in 2021; Hanford ARC was the contractor, and PCI’s planning team secured all necessary permits. About 1,000 plugs of native rush and sedge were planted along the newly fenced out creeks by Students and Teachers Restoring a Watershed (STRAW), a program of Point Blue Conservation Science that brings ecological education and opportunities to participate in restoration to local schools.

Willows planted as part of the sausal restoration will be assessed each year for five years after planting. In summer, survivorship and growth of willow plantings will be recorded and general observations of plant health noted. In May of 2022, the sausal area appeared lush and green, and the stabilized waterway showed no signs of erosion.

PCI’s construction crew continues to care for the property by removing invasive thistle, mowing firebreaks, and weedeating around the plantings. When crew members arrived on-site in May of this year, the property’s inhabitants — and original weedeaters — insisted on checking out their equipment.