Palouse Slough Internal Infrastructure Upgrade Development

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. The lower Palouse basin has a series of tide gates, including a primary gate at the Haynes Inlet confluence and multiple internal tide gates to protect the upstream properties from tidal inundation. Since 2019, the Haynes Drainage District (HDD) has been working with CoosWA to restore fish passage, habitat connectivity, productivity and water quality in the Palouse basin and Haynes Inlet. The primary gate controls the hydraulics and fish passage to the entire basin, so upgrading the primary tide gate before upgrading the internal tide gates and implementing additional restoration projects was necessary. The focus has shifted to the undersized internal tide gates that function poorly, reduce water quality and flow conveyance, and restrict fish access. The main objectives for this project are to 1) design 5 internal tide gate upgrades, 2) secure all necessary permits for construction, and 3) draft construction bidding documents.

The high quality spawning habitat of the Palouse subbasin paired with the productive estuary increases the potential coho productivity throughout all lowland subbasins in the Coos (Coos SAP, 2022). The current lack of access to rearing habitat is a major limiting factor that drives marine survival and the productivity of Oregon Coast coho. More than 50 acres of habitat in Palouse is under 3ft in elevation, and the majority is disconnected by poorly functioning internal tide gates.These internal tide gate upgrades will build upon the primary tide gate upgrade to restore connection to 50 acres of floodplain habitat, increasing the rearing productivity potential in the basin and yielding an estimated increase in adult salmon returns of 400 fish annually (Nickelson 2012).The average tidal flux will be increased further in the lower Palouse basin with upgrades to the internal tide gates which will ultimately allow for more natural conditions in Palouse Slough and improve tidal water and habitat quality as well as forage species that accelerate growth and survival rates for salmonids. Currently, stream reaches experience increased temperatures due to closed tide gate doors creating stagnant flows in the streams. The side-hinged mitigators will promote tidal mixing upstream of the tide gates and will restore a more natural salt-fresh water ecotone that stimulates growth, provides refuge, and promotes individual survival and population fitness.