Seattle Public Utilities Mami Hara, General Manager/CEO

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Reservoir Management and Research

Chester Morse Lake

There are a number of management and research projects in and near Chester Morse Lake.

The primary purpose of this component of the HCP is to evaluate the potential for using water stored below the natural gravity outlet of Chester Morse Lake (termed “dead storage”) and to assess potential environmental impacts of this significant operational change in reservoir management.

Water Storage Challenges in the Cedar River

Overflow Dike1

The Overflow Dike looking toward Masonry Pool

The reservoir complex in the Cedar River Municipal Watershed consists of two bodies of water connected by a narrow outlet channel. Water from the larger body, Chester Morse Lake, flows through the outlet channel, passes through (or over) the Overflow Dike (a small control structure) and into the Masonry Pool, which is the smaller body of water immediately upstream of the Masonry Dam.

When water levels are highest, the two parts of the system form a continuous reservoir, controlled by the Masonry Dam. But when water levels decline (typically through summer and fall), the two bodies of water can be operationally separated (i.e., Masonry Pool lower than Chester Morse Lake), using the Overflow Dike as a secondary control structure between the two.

Under drought conditions the level of Chester Morse Lake may decline to levels at which gravity flow at the outlet from the lake is not sufficient to meet demand for municipal water supply or provide instream flows adequate to protect fish in the lower reaches of the river. In order to provide enough water in the Cedar River to meet water supply and instream flow needs under these conditions, water must be pumped from deeper portions of the main lake into the Masonry Pool. Historically, this block of water has been accessible by use of a temporary, barge-based pumping system and only deployed during emergency drought conditions.

In order to have more flexibility and reliability in using water stored in Chester Morse Lake, the City evaluated the feasibility and practical aspects of building and implementing a new pumping system for moving water from Chester Morse Lake to the Masonry Pool when the water level in the lake falls below the natural outlet. After considering numerous different designs, the City chose a system that combines land and barge-based components. Construction of the new pumping system commenced in 2015, and when complete will more readily allow use of water below the lake’s natural outlet.

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Bull Trout Spawning Impedance Assessment Study

Cedar River delta

Cedar River delta at Chester Morse Lake during very low water levels.

Bull trout, pygmy whitefish, and rainbow trout all migrate into tributaries of Chester Morse Lake to spawn during fall, winter, and spring months, respectively. If water levels are excessively low during any of these spawning periods, fish could be adversely affected by impeded upstream passage or warm water temperatures at low stream flow levels.

The broad delta fans of the Cedar and Rex rivers have a steep face, or foreset, at their downstream edge where the streams flow into the lake. If the water level of the lake drops below the top of the foreset, the streams flowing down the steep slope of the now exposed foreset may become impassable to fish swimming upstream to spawn. Such a situation could potentially develop under drought conditions when lake levels can become extremely low.

A geomorphological field investigation was performed in order to better understand and assess whether the configuration of the delta fans, particularly the steep delta foresets, might present a partial or complete physical barrier for spawning fish moving into lake tributaries. This study assessed the current geomorphological characteristics of the delta fan, including topography, slope of foreset, and type and depth of substrate materials. Based on this information, the potential for a passage barrier occurring under various fill and drawdown regimes and stream flow conditions was evaluated. The study found that, initially, downcutting could be a primary cause of barrier formation (i.e., steepen the foreset); however, stream flows could subsequently reconfigure the channel, thereby reducing the foreset gradient and restoring passage.

Results from the study (Chester Morse Spawning Impedance Study pdf ) indicate that the formation of such a barrier in any given year is a possibility, but there is only a small likelihood of this barrier persisting on either the Cedar or Rex delta in successive years. The interaction of extremely low lake levels and low volume or velocity streamflow within the spawning timeframe has the potential to create a barrier on either river system until streamflow increases sufficiently to breach or reduce the slope of such a physical passage barrier, presumably via continued downcutting action.

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Bull Trout Passage Assistance Plan

In addition to the geomorphology study described for the Bull Trout Spawning Impedance Assessment, other information was collected to determine whether or not a bull trout passage assistance plan would need to be developed to address fish passage when “dead storage” is being utilized and pumping is occurring. Several HCP studies assessed fish behavior and movements in the lake and tributaries, and additional geotechnical information was collected as part of the planning and design phase of the new pumping system.

The results of these studies will be used to assess possible impacts on adfluvial fish populations from use of “dead storage”. If the combined results of these studies indicate that either the present or potentially altered morphology of the delta foresets at low lake levels poses a significant barrier to bull trout during spawning migrations, the City will develop a bull trout passage assistance plan. The assistance plan can be implemented when the reservoir level is extraordinarily low, whether or not the dead storage project is in operation.

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Bull Trout, Pygmy Whitefish and Rainbow Trout Studies

ygmy whitefish spawning swarm

A pygmy whitefish spawning swarm in Cedar River above Chester Morse Lake.

Concurrent studies of bull trout, rainbow trout, and pygmy whitefish behavior and movements were conducted in the reservoir and in selected streams. These three species occur together in the reservoir complex and tributary streams. Bull trout are federally listed as threatened. Pygmy whitefish are a major prey species for bull trout, and little was known about their ecology. Rainbow trout (especially younger age /size classes) provide an alternative food source for bull trout. These studies provided information on species distribution and life history, seasonal movement patterns, spawning behavior, response to water temperatures in the reservoir system, effects of inundation on egg and alevin survival (bull trout), and growth and productivity for each of these species.

Such studies provide information for interpretation of fish behavior in the reservoir system under current conditions and also form the basis for predicting potential impacts of future changes in operating regimes, such as the use of “dead storage” with the implementation of the new pumping system. The information provided by these investigations, as well as potential future research and monitoring efforts, serves as an invaluable resource to better inform SPU biologists and water managers on the dynamic inter-relationships between adfluvial fish species and reservoir management.

For the unique spawning movements and behavior of bull trout, view the poster (pdf), presented at the American Fisheries Society Conference in September 2011.

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Delta Plant Community Monitoring

deltaPlantCommunities 

There are four main community types present in the Cedar delta.

When the new pumping system is implemented, seasonal fill and drawdown regimes and other periodic water level fluctuations in the reservoir will likely change relative to past operations. This may include both lower drawdown levels at some times of year and higher lake levels as a result of greater water storage in the reservoir at other times. Such operational changes in reservoir levels could significantly affect plant communities on the deltas of the Cedar and Rex Rivers and smaller tributaries and in perimeter areas of the Masonry Pool. The deltas support an extensive and diverse wetland plant community system that is important habitat for a variety of flora and fauna.

As part of the Cedar permanent dead storage project, the City monitored changes in species composition and distribution of delta plant communities, evaluating effects of past reservoir management on the delta plant community characteristics, and modeling the potential effects of new reservoir operating regimes. The aquatic plant community was surveyed in 2007 and compared with previous studies (view report).

A 2013 published paper characterized the delta plant communities and examined the distribution and hydroperiods of dominant species in response to changing lake water regimes:

  • Chapin, D.M. and D.K. Paige. 2013. Response of delta vegetation to water level changes in a regulated mountain lake, Washington State, USA. Wetlands 33:431-444.

From 1989 to 2007 forested areas have decreased, open water and mud areas have increased, with variable changes in herb and shrub communities.

Based on the results of these surveys and modeling exercises, more informed decisions can be made as to the need for additional future monitoring of these delta plant communities, given that reservoir operating regimes may change substantially when use of dead storage becomes more frequent.

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Common Loon Nesting Habitat Monitoring

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A loon nesting platform about to be deployed.

The City is documenting changes over recent decades in the distribution and availability of suitable nesting habitat for common loons in the reservoir complex. This information will be used to evaluate the potential impacts to nesting habitat conditions that may result from future changes in reservoir operating regimes (e.g., use of dead storage, increased fill levels). The study will determine if impacts to vegetation, such as recession and reestablishment of willow vegetation, might have potential negative impacts to loon nesting habitat and behavior. Common loon nesting surveys are conducted annually, and if prevailing reservoir levels and seasonal timing are conducive to favorable nesting conditions, SPU staff deploy artificial nesting platforms.