Cedar River Peak Flow Management

Science Center Objects

The Cedar River watershed provides two-thirds of the water supply for the greater Seattle metropolitan region, in addition to being home to numerous federally listed salmon species. The City of Seattle, through Seattle Public Utilities (SPU), works closely with the Cedar River Instream Flow Commission (IFC) to adaptively manage flows on the Cedar River. Seattle operates its water management facilities in the Cedar River to meet two primary objectives: delivery of high quality drinking water to its customers in the Seattle metropolitan area and protection of aquatic and riparian habitat in the mainstem Cedar River and Chester Morse Lake. Water management activities also strive to consider flood protection needs, water delivery to Lake Washington/Lake Washington Ship Canal, and hydropower production at the Seattle City Light Cedar Falls Hydroelectric Facility.

To help the City of Seattle manage high-flow releases during periods of heavy precipitation, the USGS is completing a geomorphic study of the Cedar River. In particular, the USGS is working with the IFC and SPU to develop a conceptual model relating Cedar River geomorphology and aquatic ecology.

9722-DZ000- Geomorphic and hydrologic study of peak-flow management on the Cedar River - Competed FY2013

Problem - The Cedar River in western Washington drains a 186 square mile watershed on the western slope of the Cascade Range (figure 1). The upper 123 square miles are reserved as a protected watershed from which the City of Seattle gets part of its public water supply. The infrastructure for the water supply is managed by Seattle Public Utilities (SPU) and a key feature is Chester Morse Reservoir, which holds 37,200 acre-ft of useable capacity at full pool elevation. Water is diverted from the river for municipal water supply at the Landsburg Diversion Dam, located 13.6 miles downstream of Chester Morse Reservoir. Changes to the hydrologic regime from flow regulation related primarily to the management of Chester Morse Reservoir have affected the riparian and aquatic habitat along the Cedar River corridor. Because the river remains a productive area for salmon, trout, and other species, water-resource managers require detailed information on how best to manage high-flow releases during periods of heavy precipitation in order to provide flood protection while mitigating negative impacts on the fish populations and promoting river health.

Objectives - The overall objective of this study is to define the geomorphic framework and riverine habitat of the Cedar River to better understand river response and habitat health to different flood management practices. A key sub-objective is to determine the geomorphic response of the Cedar River to different combinations of flood magnitude and duration as well as to investigate the magnitude of salmonid-redd scour as a function of flood magnitude and duration.

Relevance and Benefits - This study addresses the effects of flooding on important habitat for endangered salmonids. The study contributes to the goals of the USGS strategic science direction “Understanding Ecosystems and Predicting Ecosystem Change,” as identified and described in the Strategic Science Plan of the USGS (U.S. Geological Survey, 2007).

Approach - The broad approach is to use geomorphic and habitat data to determine the relations between geomorphic features and habitat as a function of peak-flow characteristics. River geometry, geomorphic data, and additional habitat data will be collected to create a numerical model that will simulate geomorphic response in a reach of the river. Scour data will be collected to refine the relation between scour and flow and thus improve the model. Finally, the model will be used to evaluate the degree of geomorphic change and scour related to various flow magnitude and duration scenarios.