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Cedar and Green Rivers

The Muckleshoot and Tulalip tribes fish these two rivers that supply water for the cities of Seattle and Tacoma. Droughts in 1987 and 1992 (the latter occurring during this DPS) sensitized these communities to the limits on water supply even in this northwestern rain forest.

The Cedar and Green Rivers never flow together. The Cedar River drains about 188 square miles south and east of Seattle. Its headwaters are on the western slope of the Cascade Mountains, from which it flows westward into Chester Morse Lake. Seattle draws about two-thirds of its municipal and industrial water from this lake. From there, the Cedar flows through Renton into Lake Washington. The Lake is used to operate the Corps of Engineers’ Hiram Chittendon navigation locks (connecting Lake Washington to Puget Sound). The Cedar River provides about 70% of the total inflow into Lake Washington.

The Green River basin is south of the Cedar and drains more than twice as much area (about 483 square miles). Like the Cedar, the Green starts on the western slopes of the Cascades and flows west. The Corps’ Howard Hansen dam impounds up to 106,000 acre feet in the upper part of the basin, before the river flows through the Green River Valley, settled now by the communities of Auburn, Kent, Renton and Tukwila, where it finally flows into the Duwamish River, which in turn empties into Elliott Bay in the city of Seattle.

The Corps built Howard Hansen in 1962 to provide a 100-year level of flood control, water supply for the city of Tacoma, irrigation, fish conservation and pollution abatement. Commercial fisheries join the Muckleshoot and Tulalip tribes in a harvest of salmon and steelhead trout. Releases from Howard Hansen are adjusted to supply sufficient instream flows to maintain dissolved oxygen levels and sufficient depths to keep fertilized fish eggs laid along the riverbank covered.

The region was ill prepared to meet this goal when drought occurred in 1987. At this time, existing guidance for management was either limited or outdated and most personnel lacked experience in handling water shortages. Thus, it was difficult for agencies to resolve concerns, come to consensus on an appropriate course of action, and respond to drought in a timely manner. This experience clearly demonstrated that improved mechanisms for interagency coordination during drought were needed.

Following the 1987 drought, several efforts were made statewide to improve regional drought preparedness. A state drought contingency plan was developed and drought relief legislation was enacted. The Seattle Water Department (SWD) and Tacoma Water Division (TWD) initiated studies to examine ways of improving water use efficiency. Structural changes were made to the system to increase the quantity of water that could be delivered. When the DPS was initiated in 1991, stakeholders were identified and encouraged to become involved. 

The Cedar and Green River Basins

Figure 4: The Cedar and Green River Basins

Circles of Influence

Circles A and B in the Cedar and Green DPS were drawn from the Corps’ Seattle District, the University of Washington, the city of Tacoma, the Muckleshoot Tribe, and the Washington Departments of Ecology and Fisheries. Circle C included the other cities in the region, including Seattle, county governments, and the Puget Sound Regional Council of Governments.

The Cedar and Green River basins are distinct hydrologically, but in almost every other way they are one. Most of the principal participants in the Cedar- Green DPS have a direct stake in each basin. The Corps Seattle District manages structures on both basins, although its role is much greater on the Green. The Muckleshoot and Tulalip tribes fished both rivers for centuries, and, for the most part, the same state Fisheries and Ecology staff monitor both rivers. Although Seattle currently obtains no water from the Green, and Tacoma none from the Cedar, the cities have discussed creating an intertie between the two river basins to reduce drought vulnerability. Moreover, because the two cities each deal with the Muckleshoots and Tulalips, and with the same state agencies, each is attentive to the others water management programs.

Changes Resulting from this DPS

The Cedar basin model still awaits review and endorsement by the Seattle Water Department, but the contributions from the DPS have already been successfully used on the Green. The Green River basin model was first used to help agencies establish an agreed upon policy for the refill of Howard A. Hanson reservoir during March 1993. During the spring of each year, the Corps selects a refill strategy for Howard H. Hanson Dam. Their primary objective is to refill the reservoir to achieve 98% reliability. Traditionally, the refill strategy was developed independently by the Corps, without explicit consideration of the interests of other stakeholders. However, this strategy can significantly impact the welfare of different anadromous fish species at various life stages. It can also potentially impact the water supply situation later in the season. Because of these impacts, the process for establishing a refill strategy has become more open, and an opportunity for interagency policy dialog is now provided at an annual “refill meeting.” 

Typically, representatives from the Muckleshoot Indian Tribe, Washington Departments of Ecology, Fisheries and Wildlife, the Tacoma Water Division, and U.S. Fish and Wildlife Service are present. This meeting enables agencies to jointly assess the current water supply situation based on precipitation, runoff, snowpack and temperatures. It also provides an opportunity for fisheries agencies to suggest target instream flow levels to protect resource needs. The process of establishing a refill strategy is typically very time consuming, occurring over a period of several weeks.  When modifications are suggested to the Corps initial proposal, the Corps must carefully examine their potential impacts of a proposed change. This had been done using a large mainframe model. Corps personnel reported that typically, only a few target instream flow scenarios or release strategies would be analyzed, due to time constraints. Furthermore, there was no automated approach for testing system sensitivity to different instream flow target levels.

This year, the DPS Green River basin model was used as a tool to facilitate interagency policy dialogue. Prior to the initial refill meeting several runs of the model were made to assess the impacts of:

  1. Different instream flow targets.
  2. Different refill start dates on reservoir refill reliability.

This output was translated into a histogram to convey the potential implications of different potential policies to stakeholders. A wide range of instream flow target scenarios for the spring and summer months from agency comments at this meeting were tested using the model. The model was run over the historic record to examine the potential impact of these policies on refill. During the interagency working group meeting that followed, stakeholders used the model to fine-tune the most promising policies by iteratively testing the impact of their modifications. In this way, they were able to develop an agreed upon management strategy within a few hours.

Overall, the model greatly facilitated the process of establishing a refill strategy. Corps representatives were extremely pleased with the model’s ability to answer questions of concern to stakeholders during this process.  They reported that use of the object oriented model offered several benefits in comparison to previous years: 

  • First, it enabled a greater number of scenarios to be investigated and increased the amount of fine-tuning that could be done;

  • It provided participants with access to the entire historical streamflow database;

  • It enhanced stakeholder insight to system sensitivities and the relationship between proposed policies and their likely impacts;

  • Finally, it enabled them to come to a consensus on an appropriate strategy in a straightforward manner. Because of these benefits, the use of DPS Green River basin model to facilitate refill strategy development is likely to continue.

In August 1993, about twenty regional water managers representing the city of Tacoma, the Muckleshoot tribe, the state of Washington (Departments of Fisheries and Ecology) and the Corps’ Seattle District used a computer model of the Green River water management system to simulate a drought so realistically that it was termed a Virtual Drought. These parties are now formalizing an agreement to extend the collaborative efforts of the DPS into a permanent regional water management group. Efforts to do the same on the Cedar are frustrated by the fact that the city of Seattle is about 15 months behind other study participants in reviewing and correcting the model of their system.

 This study was especially useful in demonstrating the importance of getting critical players to accept a collaborative process. Most of the same players that enthusiastically accepted the DPS process on the Green were key players on the Cedar. The same modelers and managers were used on both basins. But the city of Seattle’s failure to review a Cedar River model effectively stymied adoption of a collaborative approach on the Cedar.