DESCRIPTION
The North Yakima Conservation District proposes to modify the
Naches Cowiche Canal siphon, which creates a low flow barrier to anadromous and
resident fish during periods of low flow in Cowiche Creek. The project will
replace the aging 4x4 ft. square cement siphon with a low head polyethylene
pipe approximately 3 ft. below the streambed. The City of Yakima water line
approximately, 20 ft. upstream of the siphon, will also be addressed. The
streambed will be regraded through the site to a more natural slope using a
constructed roughen channel approach allowing for improved fish passage, flood
flow, and sediment transport. There will also be an opportunity to modify the
Naches Cowiche Canal and on-farm infrastructure to accommodate irrigation
demand from the first two users on the canal. This would facilitate work to
create a new water right off the Naches River at Nelson Dam and allow their
existing Cowiche Creek water right at the Garretson Diversion (approximately 2,000
ft. upstream) to be left instream to benefit flow.
The North Yakima Conservation District (NYCD) used this grant to correct a fish passage barrier in lower Cowiche Creek in Yakima, WA. The Naches Cowiche Canal (NCC) Association siphon flowed under the stream through a concrete box siphon. The siphon had become exposed over time as the channel cut down. During low flow prior to construction, the stream flowed over the siphon as shallow sheet flow then dropped more than 12 inches, creating a barrier to juvenile salmonids. NYCD replaced the existing siphon with a new 46-inch plastic pipe placed at a lower elevation to allow for potential downcutting in the creek after project completion.
Installation of the new siphon was followed by removal and replacement of the City of Yakima’s waterline, approximately 20 feet upstream of the siphon. Prior to construction, the waterline had also become exposed due to channel downcutting. It was not classified as a fish passage barrier, but modification of the downstream siphon and the channel required modification of the waterline to accommodate regrade of the channel and eliminate the possibility of the waterline becoming a barrier in the future. NYCD replaced the waterline with an 8-inch steel pipe, lowered down in the streambed to prevent exposure from potential downcutting. Finally, NYCD reconstructed the streambed from the downstream end of the project area, up to the downstream end of Powerhouse Rd Bridge.
Construction was complete in March 2018. The next day the supervisor of the contracting company observed an adult coho swimming up and through the newly completed channel to spawn upstream. NYCD reseeded and planted all disturbed areas with native trees and shrubs in winter 2019 and mulched with certified weed-free straw. Site maintenance is scheduled to occur for the next two years, including weed control, plant maintenance, and watering.
Construction costs exceeded the original cost estimate; NYCD secured additional funds from project partners to make up the deficit. The goal of the project was to create a more natural stream slope which allows for improved flood flows, sediment transport, and passage for steelhead, coho, and juvenile Chinook salmon.
