Passive restoration through flow augmentation: the lower North Fork Skokomish River

Year: 2020
Presenter/s: Florian Leischner
Symposium Session: 2020 - 01 Protecting critical flows
Topics covered: fish passage, fish-cutthroat, fish-salmon, fish-steelhead, hydraulics, instream structure (culvert/bridge/dam), lessons learned, monitoring, and stream


acoma Power completed construction of the Cushman Hydroelectric Project on the Olympic Peninsula in 1930. The Project includes two dams, two powerhouses, and associated facilities on the North Fork of the Skokomish River, which drains into Hood Canal. The Project first impounds water in Lake Cushman, a 4010-acre reservoir downstream of Staircase Falls, and then conveys it by way of the Cushman No 1. dam to the smaller (150-acre) Lake Kokanee, impounded by Cushman Dam #2. From Lake Kokanee, Tacoma Power either diverts water through a tunnel to a second powerhouse on Hood Canal, or releases it to the lower reaches of the North Fork Skokomish River.
Historically, Tacoma Power diverted nearly all of the flow from Lake Kokanee to the powerhouse on Hood Canal, disconnecting the lower North Fork from the basin hydrology. Base flow was less than 100 cfs year-round, with minimal planned variability and large flows (greater than 1,000 cfs) largely limited to one spill event per year (on average). This was in sharp contrast to the average annual inflow of 519 cfs entering the system from the upper North Fork, with peak flows of up to 27,000 cfs. Tacoma Power maintained this hydrology for over 70 years. In combination with disconnected wood and sediment input, this flow alteration reduced the quantity of in-stream habitat in the lower North Fork through both reduced inundation and habitat simplification.

Tacoma Power received its new federal operating license for the Cushman Project in 2010. Much of the controversy during the relicensing proceeding concerned the amount of flow diverted from the river. Since relicensing, Tacoma Power has increased its base flow to 120 to 300 cfs, and has incorporated additional temporary releases of up to 1,050 cfs in response to winter storm events to create and maintain in-stream habitat below the Cushman No. 2 dam. One of the desired outcomes of flow augmentation is to support passive restoration of in-stream habitat in this reach for the benefit of fish populations, including the three stocks listed under the ESA: Puget Sound Chinook, Bull Trout, and Winter Steelhead.

Since 2012, Tacoma Power has been conducting intensive annual stream and salmon monitoring in the lower North Fork to evaluate the effectiveness of flow augmentation on habitat quantity and quality. It has been measuring the hydromorphological effects as well as the response in habitat and fish to the increase in discharges. This paper will present some preliminary results, specifically channel and habitat responses.