The confluence of the Sandy and Salmon Rivers represents one of the largest and most potentially productive restoration opportunities in the Sandy basin. Disconnected by levees for over 50 years, an initial opportunity analysis and conceptual design process in 2011 identified the confluence floodplain as the highest potential restoration opportunity in a 13-mile reach between the former Marmot dam site and the Zigzag River.
The floodplain extends nearly a mile. It encompasses approximately 418 acres. Several historic side channels and backwater ponds are currently disconnected from the Sandy except in extreme high flows. Like much of the Sandy, the confluence floodplain is isolated behind levees built after the record 1964 flood. There is an approximately 1000 foot levee bending around the upstream portion of the floodplain, and a 3000 foot levee separating the lower floodplain and its confluence with Little Joe Creek from the mainstem Sandy. Air photo records indicate the channel had occupied the floodplain historically, creating diverse aquatic and riparian habitats. The floodplain width between the river and Barlow Road ranges from 150 to 450 feet, and is owned primarily by the Bureau of Land Management and Clackamas County. Approximately 10 acres in the upper end of the area are divided into 6 private parcels.
Previous conceptual designs considered several options including:
- Partial and total levee removal
- A number of potential side channel reconnections
- Placement of large wood structures
- A maximum restoration scenario in which the county road would be relocated.
Due to the complexity of hydrology at the confluence, the potential impacts to Barlow road and a bridge immediately downstream, and the large volumes of levee material that restoration might require, construction cost estimates ranged from $1.6 million to nearly $5 million.
Sandy River Basin Partners restoration projects completed since the Sandy-Salmon confluence conceptual design have developed alternate approaches to low cost, build to fit restoration actions on public land. The updated design approach offers the possibility of significantly improving project efficiency and lowering costs. Partners’ projects on the Salmon River have also developed methods of redistributing levee material on site to reduce removal costs and add efficiency during construction.
The 2011 study indicated that the Sandy is working its way through the upper levee, as evidenced by sand deposits on the high surface behind the levee. Portions of the levee on its upstream end have continued to deteriorate. This potentially risks a passive breach through naturally erosive intrusions into the 1964 levee. The structure is not a registered flood control structure nor maintained by the Army Corps of Engineers.
Site-specific analyses are needed to determine which of the suite of options will be feasible. It is possible that over time the Sandy may continue to work its way back into the floodplain and may be able to restore flood stage processes and associated habitat. It is unlikely that the Sandy will be able to unwind the lower levee through natural processes. Implementing a project including a levee breach or a partial levee removal provides the chance to reconnect floodplain processes with a lasting impact on salmon in the river.