BACKGROUND AND PURPOSE
On May 10, 2006 the City of Bend hosted the first of five meetings of the Mirror Pond Technical Committee (Committee), chaired by City Councilor Jim Clinton. A list of the Committee members is included in Attachment A. The purpose of the Committee is to develop a better understanding of the options available to address sedimentation in Mirror Pond, and to provide policymakers advice based on sound science. The Committee sought to develop a technical framework within which the community and its decisionmakers could create preferred solutions.
The City Council provided a directive to the Committee at the joint June 7, 2006 meeting: “Do not do away with Mirror Pond.” The Council also indicated that it would like to have the Committee investigate the factors that contribute to the sedimentation, and examine possible methods of improving stream health. The Pacific Power (PPL) dam, located just downstream of the Newport Bridge, creates Mirror Pond. This directive assumes that the dam would be maintained. The purpose of this report is to summarize the considerations and recommendations of the Committee and to describe the Committee’s understanding related to the causes of sedimentation and Mirror Pond management. In addition, this report summarizes potential regulatory requirements.
Committee members concluded that it was beyond the purview of this Committee to address solutions to upstream sediment generation in detail, because other groups in the basin—such as those involved in the recent Water Summit sponsored by the Deschutes River Conservancy—are already working on such missions. The purpose of our Committee was to discuss the underlying causes of sedimentation and provide advice on potential management options that
should be explored further. Also, the Committee did not address project funding.
UNDERLYING CAUSES OF SEDIMENTATION—COMMITTEE FINDINGS
The Committee came to consensus on the following points regarding the Council’s request to confirm the underlying causes of the sedimentation within Mirror Pond.
- Committee members agreed that understanding both regional and site-specific river hydrology was important in formulating good decisions for Mirror Pond.
- Regional Hydrology. In its natural state prior to the creation and operation of manmade dams, the Upper Deschutes River was characterized by incredibly stable flow due to its connections to the groundwater system of the Upper Deschutes River basin. These groundwater flows provide more than three-quarters of the total stream flow for the entire Deschutes river basin (Gannett, et. al., 2000). As a result, large variations in peak flows were historically absent from the Upper Deschutes for three reasons: (a) the immensity, both vertically and laterally, of the groundwater system; (b) the high recharge rates and thus high groundwater discharge rates as a result of the excellent permeability of rocks near the surface; and (c) the great storage capacity of the system to absorb peaks in recharge rates (Ibid, 2000).
- Site-Specific Hydrology. Sound management of Mirror Pond cannot be accomplished without some knowledge of the hydrology of the Deschutes River through this reach. The river bottom through Mirror Pond is not smooth and rounded; the terrain varies because of the effects of the dam and varying currents within the river and the natural meander of the river as it enters and continues throughout Mirror Pond. In general, physical structures within a river—such as vegetation, sediment bars, pools (slow moving, deeper areas of streams, which are often used by fish for shelter and rest), riffles (shallow zones with fast flowing water), the channel, and meanders—all perform functions in the river. These functions include water and sediment transport, storage and transport of floodwaters, as well as the development of wildlife habitat and plant communities (Riley, 1998).
- Mirror Pond is filling with sediment for two main reasons—the upstream production of excessive sediment and its deposition in Mirror Pond.
- Production. Whereas the natural action of water in rivers tends to produce some sediment, several analyses have postulated that the operation and management of the Federal Deschutes Project, and the historical use of high-speed powerboats upstream have contributed to increased sediment production (USBR, 2003; UDWC, August 2003; Garvin, et.al, 1977; Winzler & Kelly, et al., 1981; UDWC April 2003; Dempsey, 2006). No-wake speed limits on powerboats have since been enacted upstream, although there are questions about their effectiveness in reducing bank erosion. With regards to the Federal Deschutes Project, releases from Wickiup Reservoir vary from a managed peak of 1,700 cfs during the irrigation season to 30 cfs in the fall and winter (UDWC, September 2004, quoting the Oregon Water Resources Department). This managed flow regime differs dramatically from natural conditions, and causes excessive erosion of the river banks downstream of Wickiup Reservoir Winter freeze/thaw periods loosen the exposed soil causing sloughing of the saturated soil zone into the river as irrigation deliveries ramp up (USBR, 2003). Water is released at a higher-than-natural rate throughout the spring and summer to provide water for irrigation. With higher peak flows, and limited ramping periods in the spring and summer, the net effect is that rising and falling water levels cause increased rates of erosion. There is some speculation that activities in the Little Deschutes tributary may also be contributing to sediment production.
- Deposition. The PPL dam located just north of the Newport Avenue bridge provides for a wider reach of river within Mirror Pond, thus promoting sediment deposition. The localized “problem” that we see in Mirror Pond is simply a river adjusting to a large obstruction (the PPL dam) with the resulting changes in channel dimension, pattern and profile. Because the dam has affected the profile of the river bottom, the way the river moves water and sediment has changed. As the water slows behind the dam and sediment drops out. Sediment continues to build up, and finally produces exposed sediment bars as we now see in a shallower Mirror Pond.
- Other Influences. Within the City, increased runoff of stormwater containing cinders from winter street maintenance and soil from construction sites, yards and impervious surfaces may add to the river’s sediment load. Changes to the river in the Old Mill District may influence the amount of sedimentation in Mirror Pond. When this upstream section of river was being used as a logging pond, sediment was removed by frequent dredging. The Brooks-Scanlon mill produced lumber until 1994 when it was closed by Crown Pacific Ltd., who purchased the timber company in the late 1980s. (Winzler & Kelly, 1981; Dempsey, 2006; Binus, 2005).
Although specific sedimentation data are not available, the Committee speculated that Mirror Pond would not lose its current character for at least another 5 to 10 years. After the previous dredging, sedimentation occurred at a high rate and by now is occurring at a low rate. Generally, sedimentation rate in an obstructed river is not constant: the overall rate decreases toward zero with time until equilibrium is reached.
The Committee defined Mirror Pond as the stretch of the Deschutes River bordered by the Galveston Bridge to the South, and the Newport Bridge to the North. For impact analysis of alternatives, Committee members recommended that the area examined for potential impacts should be extended both upstream and downstream.
Goals and Objectives
Based on Council direction, the Committee proposed the following primary goal for addressing sedimentation in Mirror Pond:
Implement a project that removes sediment and retains the Pond while making the Pond more sustainable and healthful than at present to the extent
The Committee suggested that components of the preferred project should recognize and/or address the following issues or objectives:
- Long-term maintenance of Mirror Pond.
- The banks along Mirror Pond.
- Water quality.
- Stormwater impacts.
- Weeds/invasive species.
- Fish and wildlife habitat.
- Valued view corridors.
- Old concrete pilings and rebar “junk.”
- The islands in the southern part of Mirror Pond.
- Recreational opportunities.
- Safety hazards to the Public.
Of note, successfully addressing the above issues would help to meet several of the community vision elements for a quality environment, as described in the Bend 2030 Community Vision Statement (June 2006).
The Committee examined the seven alternatives that had been created as part of previous efforts and brainstormed four additional partial options or potential components with respect to whether they met the above goals. Attachment B provides a description of the alternatives that the Committee reviewed, indicating which ones best meet the goals, and the rationale behind recommendations whether or not to pursue the alternative or component.
Components of a Preferred Alternative. Committee members thought that several alternatives exist that could meet both the Council’s direction of not doing away with Mirror Pond and the other goals and objectives listed above. The Committee proposes consideration of combined alternatives with various components that would allow maintaining the look of Mirror Pond in important view corridors, while helping to treat stormwater, restore wetlands, and use river flows to encourage sediment to continue downstream. By doing so, the Committee concluded that operation and maintenance costs could be minimized, habitat and water quality health could be improved, and highly valued view corridors, such as from Mirror Pond plaza, could be maintained. Passive recreational opportunities would be improved by increasing wildlife numbers and variety of species in the area. Providing a mix of components could also make for a smoother regulatory process and access to potential funding to support habitat improvements.
There are many options for a final plan within these recommendations. For example, varying the alternatives could involve channels designed to move sediment; the banks of the river could be returned to a more natural appearance and function where appropriate. Attachment C provides a list of sample performance criteria that could be used in evaluating various proposed project designs.
The Committee made the following considerations and suggestions regarding managing Mirror Pond.
- Understanding the hydrology of Mirror Pond is crucial in determining a good project. Hydrology must come first. Committee members thought that sediment sampling and river modeling studies would prove useful in determining the details of the best alternative from a technical perspective.
- A well thought-out project would provide multiple benefits and would increase the sustainability of the local solution. A sensible project and process set up to help protect resources would make the regulatory approval process much easier to navigate. Regulatory parameters must be part and parcel of the project to maximize public benefits. Cost savings could be realized as well.
- Possible solutions to the sedimentation problem could include altering the hydrology of the river within the Pond to increase the transport of sediment through the Pond. This could allow the river there to approach an equilibrium point that would result in a reduced sediment deposition rate and therefore potentially less maintenance needed over time.
- There may be better places to remove sediment than at Mirror Pond, whose surrounding lands have been built out. Using canals during sediment removal operations was discussed.
- Mirror Pond would not exist without the PPL dam. However, the aging dam is needs refurbishment and Pacific Power is not inclined to put a lot of resources into upgrade (Raeburn, August 2006). The new parent company of Pacific Power, Mid-American Energy Holding Company, is currently assessing the viability of the dam and is scheduled to make a determination by the end of 2006 (Raeburn, October 2006). Pacific Power has indicated that they will talk with City leaders prior to making any significant changes. The City will need to consider the fate of the dam as they consider the future of Mirror Pond.
The Committee also provided suggestions on a process for moving the project forward.
- The Committee recommends that the City issue a request for proposals (RFP) for a consultant team that would:
- Conduct a public process that would obtain the public’s views and preferences for Mirror Pond within the technical framework outlined in this report.
- Conduct modeling and sediment studies to quantify sediment transport and deposition (e.g., sources of sediments, makeup, depth, etc.); and to provide information on the specific hydraulics of the Deschutes River through Mirror Pond.
- Combine those ideas into a technically feasible project that would incorporate the recommended components and objectives, as indicated above, to meet the goal of removing sediment and retaining the Pond so as to make the Pond more sustainable and healthful than at present, to the extent practicable.
- Determine the most practical place to remove sediment over the long term. (e.g., in Mirror Pond, using the Pond as a sediment trap; downstream possibly in the irrigation canals where sediment could be removed during the winter once they dry out; or upstream in a settling basin.)
- Develop methods to remove sediment as well as places to dispose of the material.
- Develop cost estimates.
- For addressing the causes of the increased rates of sediment production, the Committee suggests that the City continue to work with Deschutes River Conservancy and its partners to address watershed-level problems resulting from the operation and maintenance of the Federal Deschutes Project.
Upon request by the Council, Committee members are willing to review and comment on the technical merits of consultant team proposals, and/or projects proposed by the selected consultant team.
Table 1 summarizes anticipated regulatory requirements for sediment-removal work within Mirror Pond. For a full description, see the separate Technical Memorandum entitled “Regulatory Requirements for Maintaining Mirror Pond.” Additionally, local requirements typical for a capital improvement project would also need to be met. A review of County Assessor’s records depicts property lines to the high water mark along the Deschutes River in the vicinity of Mirror Pond (Deschutes County, 2006).
Acronyms and Abbreviations:
ACOE: United States Army Corps of Engineers
DEQ: Oregon Department of Environmental Quality
DSL: Oregon Division of State Lands
EA: Environmental Assessment
EIS: Environmental Impact Statement
NEPA: National Environmental Policy Act
NOAA Fisheries: National Oceanic and Aeronautic Agencies National Marine Fisheries Service
ODFW: Oregon Department of Fish and Wildlife Service
USEPA: United States Environmental Protection Agency
USFWS: United States Department of Fish and Wildlife Service
- See in particular, Our Vision for A Quality Environment, numbers 3, 4, 7, 8, 14; Our Vision for a Strong Community, number 14.
Full Document: MP-tech-committee-summary-report