Riffles in the River
April 27, 2012
Floaters bobbing down the Upper Truckee may notice a few new bends and ripples as the river winds past Lake Tahoe Airport.
The new curves are the result of an $8 million restoration project that replaced the river’s old runway-straight channel with one that more naturally meanders. This spring marks the first flood season for the new stretch.
“We wanted to float the whole section and see what it looked like,” said South Shore resident Mike Alexander. “I was surprised to see the active erosion.”
Alexander and friend Russ Wigart have raised concerns that the project has increased the sediment load of Lake Tahoe’s largest tributary. The city of South Lake Tahoe and the Lahontan Regional Water Quality Control Board refute the claims.
The restoration of that particular stretch of river was started in 2008 with the intent to increase habitat for native fish and wildlife and reduce the amount of sediment carried by the river.
Construction crews, working through three summers, created a whole new channel with bends that would make the river overflow its banks more often than it had with the straight channel. As the water creeps over the banks, sediment that would otherwise flow downstream is captured in the flood plain.
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The old straight waterway had a greater capacity to transport sediment, according to project documents released in 2008. The river had been straightened in 1968 when the airport expanded.
Water was diverted from the old channel to the new channel in November. Flows this week topped out at more than 600 cubic feet per second Thursday and the waters flooded a few feet over the banks for the first time.
Alexander and Wigart have been measuring the turbidity, or cloudiness, of the river above and below the project. Consistently throughout dozens of samples, they found an increase in turbidity downstream of the site.
Concerned that the project could negatively impact Lake Tahoe’s clarity, they requested the city of South Lake Tahoe take its own measurements.
The city’s measurement did not reproduce their results.
Alexander’s and Wigart’s findings could be due to a number of different variables, said Robert Larsen, an environmental scientist with the Lahontan Water Board. Though there are a couple spots that do appear to need stabilization, Lahontan is satisfied that the river is functioning as the design intended, Larsen said.
“I’m happy with the way things look so far,” he said.
The city will monitor numerous metrics, including flood plain soil composition, channel width and vegetation to ensure that the new river functions as intended, said Sarah Hussong-Johnson, South Lake Tahoe’s director of engineering.
With the number of variables, it would be very difficult to measure the exact amount of sediment that flows into Lake Tahoe because of any given project, Larsen said.
What Alexander and Wigart would like is for someone or some agency to try.
“If the river doubled or tripled in pollutant loads, that, to me, needs to be measured,” Wigart said.
The widely-held assumption is that curbing river erosion and reconnecting waterways with their flood plains will help slow the downward spiral of the lake’s famous clarity. But pinning an exact number on the impact is more difficult.
“Stream restoration is not an exact science,” Larsen said. “There’s no way to directly measure the benefit or the harm of our actions.”
According to the Tahoe Regional Planning Agency’s 2011 Threshold Evaluation, released as a part of the Regional Plan Update this week, there has been little or no change over the last five years in the amount of nitrogen, sediment and phosphorous being delivered to Lake Tahoe through tributaries annually.
Down along the airport stretch Friday morning, a few birds wandered the water’s sandy edge, a rare site on the old channel’s bouldered walls. The recreated river bottom stirred bubbling rapids where fish may wait for a surface snack. Tiny sprouts of vegetation had begun to spring up through the stabilizing netting.
As time winds on, more of the restoration’s perks will begin to show, Hussong-Johnson said.
“It’s a natural system, so there is no immediate response,” she said. “You have to establish vegetation and let the river heal from the construction.”