Bathymetry of Mirror Pond

Prepared for the
City of Bend

By Joseph Eilers & Benn Eilers
MaxDepth Aquatics, Inc.
Bend, OR

June 2005

INTRODUCTION

Mirror Pond is an impounded reach of the Deschutes River located adjacent to downtown Bend, Oregon. The impoundment is created by a dam operated by PacifiCorp located immediately downstream of the Newport Bridge. Mirror Pond has a history of siltation and was dredged in August 1984 to remove some of the accumulated sediment. However, upstream bank erosion continues to supply sediment to the impoundment and Mirror Pond has once again shown signs of excessive siltation. The City of Bend contracted with MaxDepth Aquatics, Inc. to develop an updated bathymetric map of Mirror Pond to allow city engineers to assess the magnitude of sediment accumulation in the impoundment. The purpose of this report is to document the methods used to collect the updated bathymetry data and to provide the City with the results of this new survey.

METHODS

Mirror Pond was mapped on June 2, 2005. The methods consisted of using hydroacoustic techniques linked with DGPS. The positional information was derived by using a Thales ZMax RTK system; the base station was placed on Pilot Butte (GIS 031) and the rover was attached over the transducer mount on the survey vessel. The hydroacoustic unit was a BioSonics DT-X equipped with a 200 KHz single-beam transducer with a 6 degree beam angle. The rate of acquisition was set to 10 pings per second and the boat was operated at a maximum speed of about 9 kph. The boat had a minimum operating depth of about 25 cm and the hydroacoustic unit required approximately 55 cm of water to minimize near-field effects. Consequently, the effective minimum depth of mapping was 55 cm. The extensive areas of the impoundment with very shallow water required us to abandon use of fixed-line transects in favor of a more opportunistically-based survey design. The survey tracks used to acquire bathymetric data for Mirror Pond are shown in Figure 1. The shoreline for Mirror Pond was derived from a series of digital ortho-rectified aerial photographs provided by the City. The mapped surface was generated with 137,960 points using a kriging routine in Surfer® and the file used for the map preparation was a 0.5 m grid.

Figure 1. Boat tracks for surveying Mirror Pond, June 2005.
Figure 1. Boat tracks for surveying Mirror Pond, June 2005.

RESULTS
The bathymetric map of Mirror Pond for 2005 is shown in Figure 2. Morphometry of Mirror Pond derived from the bathymetric data is summarized in Table 1. Note that the bathymetric map has been prepared for purposes of evaluating sediment accumulation and no attempt has been made to identify navigational or recreational hazards present in the impoundment. For example, there are a series of metal pilings emerging above the sediment and below the lake surface located north of the foot bridge that could be viewed by some as problematic.

Figure 2. Bathymetric map of Mirror Pond, June 2005. Units are in feet.
Figure 2. Bathymetric map of Mirror Pond, June 2005. Units are in feet.

table-1

The map shows presence of a well-defined channel that extends through the impoundment and extensive areas of shoaling. The deepest areas were located immediately downstream of the Galveston Bridge indicating that current velocities remain sufficiently high to prevent sediment deposition in this area. Substrate classification was not a component of this study. However, echograms of the substrate suggest that a substantial proportion of the accumulated sediment was sand, with fine (silt to mud) in some of the very shallow areas and coarse (gravel to cobble) material in the thalweg and portions of some shorelines (Figure 3).

Figure 3. Echogram of a cross-section across Mirror Pond showing the rapid transition into the channel. The second set of echoes below the channel indicates a harder substrate. The two horizontal lines show the 1 m and 2 m depths.
Figure 3. Echogram of a cross-section across Mirror Pond showing the rapid transition into the channel. The second set of echoes below the channel indicates a harder substrate. The two horizontal lines show the 1 m and 2 m depths.

ACKNOWLEGEMENTS

This work was funded by a purchase order from the City of Bend to MaxDepth Aquatics, Inc. The project officer for the City was Mr. Oliver Fick. Steve Powell with the City kindly provided the files for the aerial coverage. We thank Matt Boyd at Watershed Sciences, Inc. for loaning us the DGPS used for this survey.

Document: MP Bathy report 2005

Report by: MaxDepth Aquatics, Inc.

Dealing with mud

OLYMPIA, Wash. – Motorists traveling between Portland and Seattle catch a glimpse of Capitol Lake as they round a steep curve on Interstate 5 here just south of the Washington Capitol campus. On clear days, the lake’s surface sparkles like a jewel hanging from the neck of the domed Capitol, which sits atop a bluff overlooking the lake.

The lake is part of a grand vision for the campus that either dates back to 1911, or to the early 1950s when a dam was built on the Deschutes River, depending who is asked.

Like Mirror Pond in Bend, Capitol Lake is a centerpiece for the community of Olympia. Its shores are circled with running trails and the state has recently completed an ambitious park project on the north end.

But like its sister river in Oregon, the Deschutes here is beginning to rebel.

As with Mirror Pond, Capitol Lake is slowly but steadily filling in with sediment. The state of Washington, which manages Capitol Lake as part of the Capitol campus, estimates that the reservoir has lost nearly 25 percent of its volume to silt and water quality has plummeted endangering fish and humans.

To deal with the problems facing Capitol Lake, state, local and tribal officials decided to develop a management strategy for the water body – similar to what city leaders in Bend have outlined for dealing with the issues facing Mirror Pond.

While the issues and scale of work are different in Olympia and Bend, it’s possible that the process used by Washington officials could serve as a road map of sorts for what happens in Bend.

Bend city councilors are expected to put together a citizen and technical advisory board this year to look at the problems facing Mirror Pond.

The problem of silt in Mirror Pond starts 60 miles south of town at Wickiup Reservoir where water is released at as much as twice the natural flow during summer months to supply farmers in the thirsty High Desert. The water picks up sediment along eroding stream banks and drops it in downtown Bend when it hits a series of dams.

The result is growing small islands of mud, the first sign of developing wetlands, have popped through the surface of Mirror Pond.

In 2002, Capitol Lake managers adopted a 10-year management plan that have helped the community take on water-quality issues and set the stage for long-term decisions about Capitol Lake.

Capitol Lake was deep enough for sail and powerboats in the 1980s. It now has to be measured in inches rather than feet in many places.

The artificially still and shallow waters of Capitol Lake have contributed to a host of water-quality issues. The state of Washington says the lake is not meeting water-quality standards for fish or wildlife. A swimming beach at the north end of the lake was closed several years ago because it was no longer safe to wade into Capitol Lake.

Last year, the state dumped thousands of pounds of pesticide into the lake in an effort to combat an invasion of milfoil, a noxious aquatic weed that can literally choke a lake to death. Managers said the weed thrived in Capitol Lake for the same reason that fish struggle – poor water quality.

In Olympia, stakeholders including representatives of the cities of Olympia and Tumwater, the Squaxin Island Tribe and the state of Washington created a committee in 1997 to study and make recommendations about Capitol Lake.

The committee was started as a way to work through the regulatory issues surrounding a new park on the shores of Capitol Lake.

“This was the opportunity to get everybody at the table and committed to doing something about the issue,” Dickison said. “We had worked with the state for years in a regulatory environment where they would need a permit and we would respond to the permit and then we would get a decision that attached some conditions. But typically, in that environment, you’re trying to stop them from doing something. You’re being reactive not proactive.”

Members of the committee said that the plan has allowed stakeholders with a broad spectrum of ideas about the future of Capitol Lake to work together on short-term and long-term management goals.

For instance, the plan clearly identifies that Capitol Lake will remain a reservoir for the next 10 years, which the state and cities of Tumwater and Olympia support.

But the committee has also started a comprehensive study of the possibility of removing the Capitol Lake dam and restoring the river and estuary – an option favored by the Squaxin Tribe.

Ryan Houston, the executive director of the Upper Deschutes Watershed Council in Bend, said the community here can learn from what was accomplished in Olympia.

Houston, whose agency has studied the problems in Mirror Pond and is currently collecting water-quality data on the Deschutes for the city of Bend, has casually followed the process in Washington. He said the emphasis that stakeholders in Olympia put on collaboration is something Bend should pay attention to as it looks at Mirror Pond.

“They invested in a community process. That is what the city of Bend would do well to do,” Houston said.

In some ways, Houston said, an open process is more important than the outcome.

“We can dynamite Mirror Pond; we can paint it blue. Whatever we decide, having a good process with good information at the table is the single lesson we could learn from those folks,” he said.

Locally, officials have identified a range of options for Mirror Pond, from dredging from bank to bank to removing the dam just north of Newport Avenue and restoring the river to its historic channel through downtown Bend.

Every option has drawbacks, either socially, politically, financially, or environmentally. Mirror Pond was last dredged in the early 1980s. But a similar dredging operation today would be difficult to push through the required regulatory hoops, said Houston. It would also be costly. Last fall, Houston estimated the cost at $2 million or more.

But there are also social factors to be weighed.

Preliminary discussions have already elicited strong emotions from residents and elected officials.

At a recent meeting of the city council’s land-use subcommittee, Councilor Dave Malkin said anyone who supports removing the dam and allowing wetlands to develop along the banks of the river would have to be from Mars.

“I believe that Mirror Pond is Bend and Bend is Mirror Pond. And to even contemplate returning that to a river channel with wetlands is completely wrongheaded,” Malkin said in an interview Friday.

There are others, including some on the council, who disagree.

Mayor Bill Friedman has said he would like the city to look at a compromise that would involve some limited wetlands development along with dredging.

That approach would help the city comply with new federal storm-water disposal rules by using the wetlands as a filter around Mirror Pond.

In Olympia, stakeholders had to deal with similar divisiveness about Capitol Lake.

In some cases, agencies within Washington State government couldn’t even agree.

The Department of General Administration, which is responsible for maintaining the Capitol grounds and buildings, has advocated the importance of maintaining Capitol Lake. The Department of Fish and Wildlife has a different view.

Fish and Wildlife Regional Director Sue Patnude has sat on the steering committee for three years. She said poor water quality in the lake has hurt the department’s efforts to bolster salmon runs.

In part because of her lobbying, the group agreed to study the possibility of restoring the estuary.

Since its beginning, the Capitol Lake committee put a greater emphasis on scientific analysis than social or political considerations, members said.

Some committee members feel the public’s voice has been marginalized.

A decision to poison milfoil last year without the approval of the city of Olympia, prompted a strong public outcry about the role of the steering committee.

In the wake of the decision, members decided to hire a consultant to look at the committee and recommend changes. Some members think those changes could lead to greater public involvement.

It’s potentially an important lesson for Bend, where city councilors are just now considering how to balance the roles of scientists and technocrats with the residents.

Bend Public Works Director Ken Fuller said city staff plan to recommend an approach that combines public input with hard science to develop a management strategy for Mirror Pond. The city council will have final say on the process, but Fuller said staff will recommend the formation of a technical advisory committee that includes residents.

“I’m hoping it gets a lot of people involved in the public process. … It will attract some attention and it should. Mirror Pond is an icon, so I think the public needs to be involved,” Fuller said.

Malkin said he is concerned that the committee might be too heavily weighted toward scientific analysis. He wants broad and strong representation from the public at large.

“I think it is absolutely imperative to that process that Bend community members be selected to be part of that advisory committee,” he said. “Yeah, we want technical folks in there, but we need committee members to represent the best interests of the community.”

Source: The Bulletin ©2005