Yuba Water implements floodplain restoration
Jul. 1—Yuba Water Agency is currently working on the third phase of a floodplain leveling and salmon habitat restoration project with the final phase expected to be completed next year. The Hallwood Side Channel and Floodplain Restoration Project was implemented to reduce flood risk and protect the habitat and spawning grounds of juvenile salmon.
Planning and construction of the project began in response to the environmental impacts of hydraulic mining in the early 1900s following the Gold Rush. The Hallwood Project was designed to remove large areas of debris called training walls which release millions of cubic yards of sediment into the lower Yuba River. The natural flow of the river and the surrounding floodplain were significantly impacted by subsequent dredging to remove the mining sediment.
The effects of hydraulic mining have led to declining fish populations, habitat loss and increased predation from non-native species of fish, said Jeff Mathews, senior project manager with Yuba Water Agency.
In order to level the floodplains, contractors with the Hallwood project had to remove the Middle Training Wall, a two-mile bank of cobblestone gravel running across the project area. Removing the training wall helped to increase natural flow and productivity in the Yuba River and improve its surrounding side channels.
"All of this is intended to increase lateral connectivity to inundated areas during times when juvenile fish and juvenile salmonids are going to be in the floodplain," ecohydrologist April Sawyer said. "This is so fish can grow bigger and avoid predators in a complex habitat before they go out to the ocean."
The project will enhance 157 acres of floodplains including 1.7 miles of perennial channels and 6.1 miles of alcoves, seasonal side channels and swales. The improved perennial channels will have access to groundwater year round, Sawyer said.
The implementation of this project has led to an increase in native salmon species including an increase in the number of juvenile salmon. The river ecosystem has also seen fewer predators for the natural fish population and fewer non-native species.
"Although the restoration actions were not specifically targeting adult salmonid spawning, we did see chinook and steelhead salmon spawning in the side channel that we constructed right after we finished up in 2020 as well as in 2021," Sawyer said.
Planning for the project began in 2012 with pre-project salmon monitoring being performed in 2014-2016. Construction to level the floodplains and create improved fish habitats began in 2019.
The debris and excess materials removed from the training walls will be moved to an aggregate processing facility and sold as additional implementation supplies to local contractors, Mathews said.
Phase one for the Hallwood Project was primarily dedicated to removing the Middle Training Wall. Approximately 1.2 million cubic yards of sediment were removed, improving 89 acres of floodplains and side channels, Sawyer said. The project transitioned to phase two in 2021. Construction moved upstream to reconnect the main river channels to the improved floodplains. Over 800,000 cubic yards of sediment were removed from the area in this phase.
The project is currently working through phase three of its construction which will continue to remove large portions of the Middle Training Wall in order to seasonally flood the Yuba River side channels. This part of the project is expected to be completed in November.
The project will reach total completion in 2023 when the Hallwood Project team removes the remaining 400,000 cubic yards of sediment from the river and surrounding training walls.
