Taking Down a Dam
Desiree Tullos stands hip deep in the Sprague River holding a collecting net inches from her face. Her long hair is pulled back in a ponytail, but the ends still tangle in the net’s meshing as she pokes at the creatures in the net. It’s a bit of a mess.
Tullos, a recent addition to Oregon State University’s Department of Biological and Ecological Engineering faculty, is examining aquatic macroinvertebrates around the decrepit Chiloquin Dam in preparation for the structure’s removal.
The dam, which is about 30 miles north of Klamath Falls, was built during World War I to provide water for the Modoc Point Irrigation District. In recent years the structure, 220 feet wide and 15 feet high, has fallen into disrepair. Its fish ladder is inadequate to allow passage, and the dam blocks endangered suckers from reaching about 95 percent of their former spawning and rearing habitat on the river. If work proceeds as planned, the dam will be removed in 2008. In the meantime, Tullos is pursuing a study that will help scientists, agencies, and communities compare the health of the Sprague River before and after the removal. Insect counts are part of a larger set of data describing all aspects of the system prior to dam removal.
“There are a lot of questions surrounding the impact of dams on the entire river system,” she says. “There’s a lot of speculation about whether we will see significant increases in fish runs. So many factors are responsible for changes in fish populations that it’s really hard to tell whether we will be able to detect a strong effect of dam removal. At this point every dam is a case-by-case study.”
Tullos is currently working with other scientists in the College of Agricultural Sciences, researchers from the Bureau of Reclamation, and the U.S. Environmental Protection Agency on field studies of the removal, as well as in the development of computer models to simulate different aspects of dam removal. She is using hydraulic models to look at the possible distribution of river sediment after the dam is removed and how this sediment may affect the environment.
During normal flows, the river carries sediment downstream in the current, creating new sandbars, changing channels, and varying depths. Damming a river results in a buildup of sediment behind the dam’s upstream wall. The buildup resembles a giant cheese wedge, tallest directly next to the dam and tapering away upstream. The wedge of sediment from Chiloquin Dam to 900 feet upstream is conservatively estmated at 45,000 cubic yards, or 61,000 tons. When the dam is removed, the models predict the channel will resemble a stair step as it moves the stored sediment downstream during high winter flows.
“The models are telling us the systems could redistribute the sediment in a way that would not cause an immediate shock to the ecosystem,” she says. “But there are other sediment questions besides redistribution. We need to think about changes to habitat, water quality, and temperature. It’s not a simple problem with a simple answer.”
In 2003, Gregory Stewart, an OSU doctoral student, helped document the removal of Dinner Creek Dam in Cottage Grove. The dam, a fairly small structure just under eight feet tall, left behind about 980 cubic yards of sediment. Stewart, who is now a staff scientist for Washington State’s Cooperative Monitoring, Evaluation and Research Committee, says that he had expected to see a large blast of sediment move downstream after the removal and was surprised when it didn’t happen.
“Instead of one large movement, we found that the sediment redistributed with the occurrence of storms and natural events,” he says. “We ended up having a series of little bleeds rather than one big burst. The episodic sedimentation of the system has been similar to what would normally occur during a storm or flood event.”
The removal of Dinner Creek Dam has so far not resulted in any large, catastrophic impacts to the creek system, and the sediment has in fact acted as a regulator for stream temperatures, Stewart says. “Dinner Creek appeared to recover very quickly, but each dam and river are going to be different.”
There are currently more than 3,500 dams in Oregon. The majority are relatively small structures, like Chiloquin, primarily used to provide water for municipal, industrial, irrigation, livestock, and rural use. The others, about 136 of them, provide hydropower to homes and businesses across the state. When the dams were built, they were seen as positive developments, providing flood control, clean energy, guaranteed water supply, and recreation. In time, however, problems appeared, including the blockage of fish passage, the alteration of flood plains and river channels, and changes in water quality.
Today, about 90 percent of Oregon’s small dams are more than half a century old and are approaching the end of the 50-year licensing period granted by the Federal Energy Regulatory Commission, says Tullos. Many of these dams are outdated and falling apart, with exposed reinforcing bar and crumbling concrete. In order for the dams to be relicensed, they must meet current safety and fish passage requirements, which could mean millions of dollars spent to retrofit fish ladders and byways. If the dams fail to meet current regulations through upgrading, they will need to be removed, as is the case in Chiloquin, Tullos says. Many of the small dams being considered for removal may be replaced with irrigation pumping stations, which often have a smaller impact on the river system than dams.
Of the more than 75,000 dams nationwide, about 500 have been removed to date. More are likely to follow in the next several years, as Congress has been asked to fund dam removal programs as part of the 2007 fiscal year federal budget. “Within the next 20 years we’re going to see a lot of small dams coming out of Oregon’s rivers,” Tullos says. “Removal could occur in two ways: the blow-and-go option, where the reservoir behind the dam is dewatered; or a staged removal where the structure is pulled down in sections.
“There are people who see dam removal as a silver bullet for fish recovery, with expectations that record runs are going to occur the season following removal,” she says. “I’m not sure pulling out dams will fulfill all the dreams. It’s rarely that easy. Other factors, including habitat quality, water quality, and watershed activities are important parts in endangered species recovery.”