Photo by Marcy Stamper,br \> Environmental engineer Chris Matson explained the new spillway design for the Chalfa Dam (also known as Wenner Lake) in the upper Benson Creek watershed.

Photo by Marcy Stamper
Environmental engineer Chris Matson explained the new spillway design for the Chalfa Dam (also known as Wenner Lake) in the upper Benson Creek watershed.

By Marcy Stamper

With rain adding more moisture to ground already saturated from snowmelt, an engineer, habitat biologists and a conservation planner took interested Benson Creek residents to a dam high in the Benson Creek watershed to explain repairs that will be made this summer. The group toured the area on Thursday (April 14).

Chalfa Dam (sometimes called Wenner Lake) is the highest of what had been a string of five lakes in the upper Benson Creek watershed. The Washington Department of Fish and Wildlife (WDFW) also owns the next lake in the chain, which has been allowed to fill with silt and become a wetland, said Chris Matson, an environmental engineer with WDFW.

The spillway on the Chalfa Dam failed in August 2014 after heavy rains caused erosion on steep slopes, where there is little vegetation left to hold soil and other debris in place.

The three lower lakes are privately owned. The owners of the middle — and largest — lake, called Rabel Dam, made emergency repairs immediately after the breach, according to James DeMay, dam safety section manager for the Washington Department of Ecology. The other two lakes have also reverted to wetlands.

Lower in the watershed, snowmelt and high runoff have carried considerable sediment into the creek and inundated fields on Benson Creek Road, where the creek is spreading out in braided channels, said Carmen Andonaegui, a WDFW habitat program manager. She said they may have to consider reconstructing the old creek channel if it poses safety risks. “This is a natural disaster, but when people live there, it’s so different,” she said.

Benson Creek residents are still grappling with the effects of the fire and flood. A group of irrigators is researching the best option for restoring their irrigation infrastructure, which was destroyed in the fire. They are weighing rebuilding the piped system fed by Benson Creek and Chalfa Dam against an alternative of converting to wells.

Short- and long-term

The Benson Creek watershed group has been looking at short-term needs to address immediate problems facing residents and roads, and at a longer-term strategy for the entire watershed. A watershed strategy would be more sensitive to environmental needs, said Terri Williams, a conservation planner with the Okanogan Conservation District who has been working with Benson Creek residents on impacts from the fire and flood.

The upper Finley Canyon area around the lakes is among the most severely burned in the Carlton Complex Fire and scientists predict it will take a long time for the high slopes to recover, said Williams.

Chalfa Dam will be drained and construction will begin as soon as there are no more concerns about flooding from spring runoff, said Matson. WDFW will construct a 40-foot spillway, 10 times the size of the old one, and install a valve that will allow them to drain the lake as needed.

Construction is expected to take up to three months, said Matson.