Palouse Tide Gate Upgrade Development: Final Design

Palouse Slough drains into Haynes Inlet in the Coos basin and is one of the highest producing coho anchor habitat streams on the Oregon Coast. Its primary tide gate consists of a collapsing, undersized tide box with two top-hinged wooden doors located under a county bridge, confounding the responsibilities for replacement and leading to a strong desire to decouple the structures. This technical assistance project was the second & final development phase for the Palouse tide gate upgrade, building upon Phase 1’s alternatives analysis study that investigated the best long-term solution for the Palouse tide gate design and the feasibility of decoupling the infrastructure. After a robust review by a team of local/regional experts, a preferred design alternative selected is a sheet pile structure with a 4-bay modular gate just upstream of the existing infrastructure.

For its size, the Palouse subbasin is one of the highest productivity coho anchor habitat streams on the Oregon Coast. The high quality and stable spawning habitat in the upper watershed within the Elliott State Forest interacts with the diverse and productive estuary system in Haynes Inlet at the Palouse confluence to release much of the potential coho population productivity. The Palouse subbasin provides productive juvenile coho salmon rearing despite considerable alterations to the middle and lower reaches of the stream, the most profound of which is the primary tide gate and the target of this project proposal. Reconnecting the diverse low to moderate gradient and off-channel flood plain habitats in the lower reaches, beginning with tide gate replacement, will further release the considerable rearing productivity potential above and below the tide gate by providing seasonal access and egress to these habitats. The current lack of access is a major limiting factor that drives marine survival (smolt to adult) and the productivity of Oregon Coast coho populations in general. The proposed tide gate replacement will upgrade the tide gate to a side-hinged tide gate with an adjustable muted tide regulator (MTR) that will allow the gate doors to be open longer that the average top or side-hinged door, increasing the amount of time migrating adult and juvenile fish can pass through the gate.

Tide gate replacement and upgrading will release hydrological function that mimics natural seasonal and tidal cycles, restoring the system’s hydrology and water quality up and downstream of the gate. Currently, lower Palouse stream reaches experience increased temperatures due to stagnant flows and highly variable levels of salinity due to holes and other leaks in the tide gate structure. Adaptive management of MTR gates at Palouse will restore a more natural salt-fresh water ecotone that stimulates growth, provides refuge and promotes individual survival and population fitness. Replacing the current tide gate with a custom MTR tide gate design and developing a seasonally adaptive water management plan will restore tidal inundation to a controlled elevation providing not only improved access and egress to off-channel rearing habitat, but clearer eco-evolutionary signals for salmon smolt emigration, significant reductions in velocity passage barriers, noxious weed and sediment control. A return to more natural tidal mixing will mediate temperatures directly upstream of the tide gate which is a lethal thermal zone for salmonids in summer months when the doors are closed for extended periods (days) during neap tidal cycles.

Greater tidal connectivity to Haynes Inlet downstream of Palouse will improve estuarine water and habitat quality for rearing salmonids as well as salmon forage species that accelerate growth and survival rates. This proposed project is the first step toward comprehensive subbasin scale restoration of the Palouse subbasin that will protect and expand this area of critical Oregon Coast coho anchor habitat.