Studying the sounds of the streams

2011-12-15T12:00:00Z 2011-12-15T12:03:27Z Studying the sounds of the streamsBy ROB CHANEY of the Missoulian missoulian.com
December 15, 2011 12:00 pm  • 

If you listen to the water hard enough, you can hear the earth move.

Poets have scribbled about babbling brooks for centuries, but none of them ever stuck a hydrophone in the current and really paid attention. It turns out, a river will tell you a lot about alluvial life if you have ears to hear.

"We realized that nobody has ever floated down rivers and listened to what they sound like," University of Montana scientist Mark Lorang said. "There's been a lot of work done on coastal environments, listening to sea lions and whales. But the sounds generated by rivers themselves - there hasn't been much work done."

That's because most people assume listening to a river would be as thrilling as a white-noise machine. And, in fact, if you stick a hydrophone in a river, that's exactly what you get.

But stick two hydrophones underwater, and you can use an acoustical filtering trick to cancel out the random turbulence, the same way noise-canceling headphones can muffle the drone of an airplane from your iPod.

Then you notice the difference between a sandy river bottom and a gravel bed. You can hear the relative silence in the eddy behind a boulder compared to the main flow of the current.

"During floods, you can really hear the sediment moving," Lorang said. "You can't see it, but in low flows it's really quiet. When the river's flooding, it's the equivalent of walking into a rock concert."

This opens up two exciting research doors: animal and mineral. On the biological side, Lorang and his fellow scientists found rivers have distinct acoustic signatures. He can tell the difference between the Middle Fork of the Flathead River, which flows mostly over bedrock, and the North Fork of the Flathead, which has a bed of cobblestones. The Middle Fork is much quieter.

Lorang suspects fish and other river critters may listen for the relative quality of a river reach when they choose where to spawn or feed. Any good angler knows trout congregate behind big rocks and in eddies. Perhaps they can hear the presence of these features even when the water's too cloudy to see them.

"Riffles, runs and pools - places fishermen would recognize - have really distinct sound signatures," Lorang said. "It depends on the turbulence in the system."

A lot more research has to be done before that theory grows legs, he added. But it could result in ways to gauge the relative health of a river, or give clues to the signals trout and salmon use to find a fish ladder around a dam instead of the channel leading to its turbines.

***

The geological side of the science may produce practical results even sooner. That's because a lot of people want to understand how a river moves everything from silt to boulders during a flood.

"The traditional way is to have a sampler sitting on the bed of a river, physically catching the sediment moving along," said UM geomorphologist Andrew Wilcox. "It can be extremely labor-intensive and dangerous. You're out there during floods, trying to find places you have to take the samples from. There are a whole set of challenges that have led to a big effort to come up with alternative ways to use technology."

German river ecologist Klement Tockner teamed up with Lorang at the Flathead Lake Biological Station to make an international stab at the river hydrophonic study. They mentored graduate student Diego Tonolla in sampling rivers in both the United States and Europe to gather a wide variety of soundscapes to compare. In particular, they explored the rivers around Glacier National Park and the Tagliamento River north of Venice, one of Italy's longest undammed rivers.

Knowing how much sediment might move where is important when you're doing things like knocking down Milltown Dam. Its reservoir on the Clark Fork River contained several million tons of mud mixed with a century's worth of toxic mine waste. Before it was drained, health officials had to be satisfied that waste would stay in place so it could be excavated - not flushed downstream to pollute new reaches.

On a more routine basis, dam operators often release water to mimic seasonal flooding for habitat improvement. But it's hard to be sure if the man-made floods are pushing sediment the way a real river would. Bet wrong, and fish spawning beds could be suffocated under a layer of silt instead of refreshed with a dose of fine gravel.

"We think we can listen to sediment transport and relate the intensity to different size fractions - cobbles, gravels and pebbles," Lorang said. That might tell when a river is starting to scour its bed, which could uncover buried pipelines like the one that burst on the Yellowstone River last spring.

Reporter Rob Chaney can be reached at 523-5382 or at rchaney@missoulian.com.

 

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