Mirror Lake Comprehensive Floodplain Reconnection

The Mirror Lake site is a 440-acre tract of Columbia River floodplain located within Rooster Rock State Park in Multnomah County. It provides nearly 20% of accessible floodplain habitat in the lower Columbia River Gorge and supports spawning populations of coho salmon and lamprey. Additionally, salmonids from every major interior portion of the Columbia River basin utilize lower Gorge floodplains (including Mirror Lake) for off-channel rearing during outmigration to the ocean, and Chinook, coho, and chum salmon, as well as steelhead, all utilize the Mirror Lake floodplain. The site is of elevated importance to outmigrating salmonids because it is located immediately downstream of the Columbia River reservoir system, which has limited rearing habitat. Within the project site, habitat conditions are degraded by historic land uses and the construction of Interstate-84, which severed the site’s upstream connection to the Columbia River and eliminated flow-through hydrology. Currently, connectivity to the Columbia River is limited to one culvert located at the downstream end of the site that allows for backwater hydrology only. The net effect is muted hydrologic processes, and limited distribution of upriver salmonids rearing at the site (fish sampling indicates they do not utilize the upstream half of the site).

The project team is conducting a formal alternatives analysis. When complete, the team will use this funding request to develop a 60% design plan set (plans, basis of design report, and cost estimate) to (1) reconnect the upstream end of the site to the Columbia River to re-establish flow-through hydrology and improve fish access, (2) reconnect one salmonid-bearing tributary to its floodplain, (3) reclaim the boat basin to provide off-channel wetland/riparian habitat, and (4) remove an abandoned culvert.

Project partners include OR Parks and Recreation Dept., OR Dept. of Transportation, Bonneville Power Administration, and Federal Highways Administration.

The projects goals are to restore floodplain connectivity, improve wetland habitat complexity and function, increase nutrient and prey export, and increase fish access. Project actions will address impaired channel structure, limited habitat complexity, reduced habitat diversity, and diminished floodplain functions, all of which are limiting factors for salmonid recovery. Additionally, the project would create desired environmental conditions which increase shallow water and side channel habitats, restore floodplain connectivity, and maintain favorable flows for migrating adults. The combined effect will result in increased prey base, increased habitat quality, diversity, and availability, improved fish passage, and expanded salmonid distribution within the site.