Hydrologists discuss Mirror Pond
二月 22, 2013
Experts suggest compromise plan to address silt buildup
By Scott HammersHydrology experts assembled by the City Club of Central Oregon said Thursday there’s no urgency to develop a plan to address silt buildup in Mirror Pond, and suggested an approach somewhere between attempting to maintain the historic pond and removing the Newport Avenue Dam could win broad community support.
Hundreds filled a lecture hall at St. Charles Bend on Thursday to learn more about silt accumulation in the downtown Bend pond, formed by the construction of the dam 100 years ago and last dredged in 1984. In the years since then, the pond has become shallower, the result of silt washing into the Deschutes River upstream and settling on the bottom in the slow-moving waters of Mirror Pond.
Hydrologist Joe Eilers — who was joined by hydrologist Gabe Williams and Upper Deschutes Watershed Council Director Ryan Houston on the City Club panel — told the audience Thursday that dredging to preserve Mirror Pond as a pond is at best a short-term fix.
By deepening the channel, dredging causes the water to move even more slowly, Eilers said, allowing more silt to fall out of suspension rather than be carried further downstream. A pond like Mirror Pond will re-silt fastest in the first few seasons after dredging, he said, reaching 80 percent of its maximum silt-holding capacity within 10 years, and 90 percent within 20 years.
“If you’re going to go the full dredging route, you might as well buy a dredge, because you’ll be back there in the not-too-distant future," Eilers said.
As of today, Mirror Pond probably has about 90 percent of the silt it can take, Eilers said, but it’s hard to know when it might reach 100 percent.
Icing during the winter has so far discouraged plants from taking root where they might turn shallows to dry land, he said, adding that even if the pond reaches its maximum silt-carrying capacity, the water should continue to flow.
Club member Jim Lussier, the former president and CEO of St. Charles Health System, asked the panelists what the long-term costs of doing nothing might be.
While Eilers focused on the cost of maintenance that is presumed to be needed on the aging PacifiCorp dam, Houston said the cost of inaction may be more abstract. Those who enjoy the views across the pond, its waterfowl, or paddling along the slow-moving waters could lose those amenities if Mirror Pond is left alone, he said.
“It’s not just the capital expenditures, it’s what do people care about," Houston said.
Specifics of the future of the dam were left unaddressed Thursday. Although not on Thursday’s panel, Angela Price of PacifiCorp was in attendance. Price declined to elaborate on how long Pacifi- Corp intends to continue operating the dam, or what might happen if her company concludes the cost of upkeep outweighs its power-generating potential.
Taking out the dam completely would have a significant impact beyond the area commonly thought of as Mirror Pond, Houston said. Removing the dam would drop water levels directly behind the dam by 8 to 10 feet, he said, and the river would find a new channel through the main body of the pond. The effect could be noticeable as far upstream as McKay Park, where Houston said water levels could drop by a foot.
In response to an audience question, Houston said many of the consulting engineers working on possible solutions for Mirror Pond are also working on the Bend Park & Recreation District’s plans to develop a safe passage through the Colorado Avenue Dam spillway, and are confident they can find a way to make both projects work together.
The dam’s removal would be the best option for fish, Eilers said, lowering water temperatures and boosting the level of available dissolved oxygen by allowing the river to move faster. He said a faster-moving river through Drake Park would also be likely to drive off the geese that have multiplied in the area over the years.
Eilers suggested a fourth option — which he dubbed “designer dredging" — might be the easiest course of action. Such an approach could involve dredging out a defined channel while building up and “armoring" some areas where silt deposition is most pronounced. Other portions of the pond could be restored as above-water-level parkland, he said, such as the shallows in the wide bow just behind the Drake Park stage.
Mike Hollern, CEO of Brooks Resources and a pond-side resident, latched on to Eilers’ description of “designer dredging." Hollern said his personal preference would be for the future pond to retain many of its present characteristics, but acknowledged that those who live closest to the water benefit most, and should contribute to a local improvement district to help pay for any work on the pond.
Hollern suggested a retaining wall backfilled with silt dredged from the pond could be used to expand Harmon Park on the west side of the river.
Houston said such a compromise could hit a “sweet spot" that could at least partially satisfy those who desire views, wildlife habitat and access to the water for recreation. Dry land for expanded parks could persuade the park district to buy in, Houston said, while developed wetlands that could help clean up the wastewater dumped into the pond by city storm drains could attract funding from the city or grants from clean water groups.
Gary Fish, founder of Deschutes Brewery, joked that the brewery would have to scrap “about 25 million pieces of printed material" depicting the pond that serves as the namesake of Mirror Pond Pale Ale, but echoed Houston’s comments about finding a middle ground between dam removal and repeated dredging.
“I don’t think anybody’s going to get everything they want, but they should get a lot of what they want," Fish said.
Thursday’s forum was independent of an effort under way by the Mirror Pond Management Board, a group formed by the Bend City Council in 2009. The management board has an online questionnaire where local residents can share what they value about Mirror Pond and the Deschutes River at www.mirrorpondbend.com through Feb. 25. In March and April, the board will be using the public input it’s gathered to develop potential plans of action, including illustrations and cost estimates.
Past estimates have placed the cost of a 1984-style dredging at between $2 and $5 million.
— Reporter: 541-383-0387, email@example.com