The silt situation: How to keep Mirror Pond clear
мар 08, 2015
Bend BulletinBy Scott Hammers
Even if the plan to replace the Mirror Pond dam and modify the pond floor to speed the water and deter accumulation of sediment becomes a reality, dredging will be a key part of pond maintenance for as long as the pond remains a pond.
Since the city and the Bend Park & Recreation District began discussing what ought to be done to deal with the sediment that’s built up since the pond was last dredged in 1984, officials working on the project have been clear — if local residents want to keep a wide, slow-moving pond in the center of the city, that pond will continue to be a magnet for silt, mud and other debris carried from points upstream.
During the 1984 dredging, crews removed approximately 60,000 cubic yards of sediment, enough to cover a football field — end zones included — more than 28 feet deep. Even so, the 1984 dredging only removed a fraction of the estimated 350,000 cubic yards of sediment behind the dam at the time. By most estimates presented by officials working on the Mirror Pond project, sediment levels in the pond are at or near where they were before they were last dredged.
Public discussions of how all that sediment has made its way to Mirror Pond have been few, but the Mirror Pond Ad Hoc Committee — the primary group working on pond issues today — has been in discussions with the Oregon Water Resources Department to see if there might be a way to cut off some of that sediment at its source.
Ryan Houston, executive director of the Upper Deschutes Watershed Council, and Matt Shinderman, a natural resources instructor at OSU-Cascades, said all rivers are constantly carrying sediment from upstream to downstream, but the Deschutes, and particularly the upper Deschutes, likely carries more than average due to the way it’s managed for irrigation. Houston and Shinderman were members of earlier committees studying sediment accumulation in Mirror Pond, and both described the mechanism by which the Deschutes picks up and moves sediment in similar terms.
It begins 55 miles upstream from Bend, at Wickiup Reservoir.
The largest storage reservoir in an irrigation system that ferries water to cropland from Bend to north of Madras, Wickiup Reservoir began operating in 1947. Filled with winter rains and snowmelt, the reservoir controls the flow of water into the upper Deschutes — less in winter to allow the reservoir to fill, and much more during the summer growing season.
Prior to the construction of Wickiup, the Deschutes was noted for its unusually constant flow from winter to summer. A 2012 research paper by the Deschutes River Conservancy stated pre-dam winter flows below the reservoir averaged 660 cubic feet per second, with summer flows averaging 730 cubic feet per second.
In order to provide water to farmers and ranchers when they needed it, Wickiup altered those patterns, and drastically.
Between 1981 and 2011, the median winter flow in the Deschutes below Wickiup Reservoir was measured at 37 cubic feet per second. The median summer flow was 1,150 cubic feet per second, and higher still in July and August.
As a result, the river flows much lower in its channel in winter, and much higher in summer.
Areas that would have remained mostly submerged under near-constant flows are exposed in winter, making it difficult for aquatic plants to take root and stabilize the soil. Exposure to the air subjects these areas to a cycle of freezing and thawing through the winter, further loosening the soil. When the river level rises during irrigation season, a portion of this soil is swept up and washed downstream.
Shinderman said human activity beyond the irrigation system also contributes to sediment in the upper Deschutes and the little Deschutes, its flows similarly managed by a reservoir at Crescent Lake.
“We’ve built docks all along the upper Deschutes, we’ve built homes, we’ve removed riparian vegetation and replaced it with sod,” he said. “There are a lot of things that happen in the upper Deschutes basin that create erosion.”
Higher summertime flows in the upper Deschutes give the river more power to nibble away at the banks along bends, Houston said, and the fine ash soil that lines much of the river is easily eroded. As the river straightens, that power is enhanced further, he said.
“Every time a river meanders and it moves side to side, it’s releasing energy and dissipating energy,” Houston said. “When you go from a meander to straight, the river has more energy, and more ability to pick up particles and move them downstream.”
Aerial photos from the area just south of Sunriver show what’s changed over the decades, Houston said. Wide oxbow bends have been cut off, with the water finding the shortest route in its journey downhill. Since Wickiup Reservoir began operating, it’s estimated the Deschutes River has grown 20 percent wider on average above Bend, he said, as much of what was once the river’s banks has been washed downstream.
In theory, it’s possible to determine what parts of the river are generating the most sediment. The composition of soils through the upper Deschutes basin varies somewhat, Shinderman said, and by taking core samples in areas with heavy sediment deposits like Mirror Pond or the Old Mill District, researchers can identify where they were likely located before being carried downriver.
Shinderman said such a study would be expensive and time-consuming, but could help pinpoint what kinds of anti-erosion efforts might be most effective upriver.
“I think it’s important we put some effort into trying to determine where the sediment is coming from,” Shinderman said. “If you don’t know where the sediment is coming from, it’s almost impossible to come up with a sustainable solution — you’re just doing a Band-Aid approach.”
While erosion along the upper Deschutes is widely believed to be the primary source of sediment in Mirror Pond, runoff within the city plays a role as well.
Much of the stormwater from the west side of Bend and downtown Bend drains into the Deschutes River, entering the river at outfall pipes primarily upriver of Mirror Pond. As part of the larger vision to rehabilitate Mirror Pond, officials working on the project have proposed finding a way to filter the stormwater emptying into the river.
Wendy Edde, stormwater specialist with the city of Bend said some filtration systems have been put in place since the pond was last dredged.
The city has 10 sediment manholes in various areas around the city that drain to the river, Edde said, essentially large underground vessels that filter stormwater through the same mechanism that allows sediment to build up in Mirror Pond. Water flowing into the manholes is allowed to rest before continuing to the river, she said, giving small particles an opportunity to sink to the bottom. Yearly, city crews inspect the city’s sediment manholes, and clean out the accumulated sediment.
Edde said before city road crews switched from scattering red cinders on icy roads to a duller shade of rock, reddish fans of deposits could be seen on the bottom of the pond at many of the outfall pipes where no filtration system is in place.
Aggressive street sweeping helps reduce the amount of material that ends up in Mirror Pond. Between July 2013 and June 2014, city crews swept up 18,187 cubic yards of dirt and debris along the roads, Edde said, though that figure includes areas of the city that do not drain to the river or Mirror Pond.
Beyond reducing the frequency of dredging, cutting back on the sediment that flows in to Mirror Pond could improve the health of fish and other wildlife living in the pond, Edde said.
“Sediment is listed as a pollutant of concern in the river,” she said. “It can clog fish gills, reduce disease resistance, and cover the bottom and destroy the habitat of small organisms that supply food to other wildlife.”
Kyle Gorman, the region manager with the Oregon Department of Water Resources, helps decide how much water is released from Wickiup Reservoir and when. Gorman said he doesn’t dispute the mechanics of how management of the flows at Wickiup Reservoir contributes to sediment accumulation in Bend.
“I think what we’re seeing now is just the change in flow regime that was historical there before the dam,” he said. (The river is) significantly different, it’s changing its shape, it’s got more energy.”
Gorman said after almost 70 years of modifying the flows in the upper Deschutes for irrigation, it appears unlikely the river will reach a point of equilibrium where it’s no longer carrying large amounts of sediment downriver. To slow the transport of sediment, Gorman said the winter and summer flows in the upper Deschutes need to be brought closer to where they were before Wickiup Reservoir was built.
Getting there could take various forms, Gorman said. If the various irrigation districts that depend on Wickiup could find other reservoirs to store their water allotment, he said Wickiup could release more water through the winter. Reducing the amount of water lost to leakage and evaporation in the pipe and canal systems that deliver water to fields and farms would mean less water would have to be sent down the Deschutes to the spots near Bend where irrigation districts draw water from the river — allowing more water to flow into the river from Wickiup in winter.
Gorman said irrigation districts, local governments, conservation groups, and others with a stake in the Deschutes River have recently begun a study of the entire Deschutes basin, from its far southern reaches to the drainage basin that feeds the Crooked River, clear to the mouth on the Columbia River near Biggs.
Sediment is only one facet of the three-year study, Gorman said, which is currently in its planning stages. But boosting wintertime flows below Wickiup Reservoir is a target, he said — and if that can be achieved, they’ll be able to turn their attention to replanting and restoring the eroding banks of the upper Deschutes.
— Reporter: 541-383-0387, firstname.lastname@example.org