WARM SPRINGS (AP) — With a clattering splash and a huge cloud of dust, a tub full of rocks dropped into the Deschutes River. Free of the half-cubic yard of gravel, the tub, dubbed a gravel-o-matic, bounced up and down on the cable that suspended it above the water.

A winch — usually used to drag trees instead — hauled the gravel-o-matic, which looked like an oil drum sliced in half, up to the Warm Springs side of the Deschutes.

There, a backhoe filled the contraption with the next load of rocks to be slung across the river and dropped into its designated place.

With 600 more gravel deliveries like that one spread out over a few days, scientists with Portland General Electric and the Confederated Tribes of Warm Springs are starting a multiyear study to look at how gravel and rocks are washed down the Lower Deschutes.

Since rocks like these serve as nest materials for steelhead and salmon, the project is designed to see if the Pelton-Round Butte dam complex, located just upstream, is blocking necessary components of fish habitat in the wild and scenic river.

After at least five floods have had a chance to wash rocks downstream, crews will gather the data. They’ll check whether hand-placed, electronically tagged rocks have moved, and use cameras mounted on weather balloons to check on the changes in the riverbed.

“We’re doing this gravel study to see, are we losing gravel from the system,” said Bob Spateholts, the aquatic habitat team leader with Portland General Electric, which operates the dam complex with the tribes.

Some biologists think that’s the case, and that the gravel that lines the riverbeds washes downstream but isn’t replenished because of the dams, said Jim Manion with the Confederated Tribes of Warm Springs.

Other biologists he’s talked with say it’s not an issue, since the Lower Deschutes has a relatively steady flow of water that isn’t likely to move rocks around very much.

The project is one of the things the power company and the tribes agreed to do as part of the federal relicensing of the dams, Spateholts said, adding that designing the study and placing the gravel probably will cost between $50,000 and $100,000.

And if the study shows the stretch of river is losing gravel, and therefore potential fish habitat, the two dam operators could have to start replenishing the supply.

Studying the movements of rocks might seem esoteric, said Ryan Houston, executive director of the Upper Deschutes Watershed Council. But where the rocks end up can fundamentally change where fish such as salmon and steelhead lay their eggs.

“The way that gravel is distributed is critically important because it’s what forms those spawning areas,” Houston said.

Some fish can spawn in larger rocks; others need smaller gravel pieces, he said. And if the river flows are washing the smaller rocks downstream, while the dams are blocking the resupply of those smaller pieces, some fish could be without sufficient habitat.

“It’s kind of an indicator of how the river is functioning,” he said.

In all, crews will distribute about 300 cubic yards of rocks, which are between 1 inch and 6 inches in diameter, to three sites between the dam and the boat ramp south of the Warm Springs Reservation, Spateholts said.

Each site is marked with stakes illustrating how deep the new gravel bed should be in order to mimic a natural river bottom, he said.

And after the next five flood events researchers will come back to remeasure the depths to see if they have changed.

The crews will also be able to trace individual stones. Some will be painted white, while others will have holes drilled into them and an electronic tag, similar to the type implanted in pets, will be glued in. Then researchers with GPS devices can track where each of these marked rocks has ended up.

They’ll also use low-elevation aerial photography. At some point, crews will come out with cameras held aloft by weather balloons to take a series of pictures, Spateholts said. These pictures, from 200 feet up, will give another view of how gravel is moving or not moving down the river.

It’s possible the gravel will just stay put, he said, since the spring-fed Deschutes River doesn’t flood as dramatically as other waterways. Computer modeling has suggested that a layer just one pebble thick has been washed down the river since the dams were built almost a half-century ago. But the study should help answer the question, either way.

“It’s important to know — are the operations of the dam causing effects to the habitat?” Spateholts said.

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On the Net:

Upper Deschutes Watershed Council: http://www.restorethedeschutes.org/

Pelton-Butte fact sheet: http://www.portlandgeneral.com/about—pge/news/peltonroundbutte/facts heet.asp