Gauging Flood Conditions Behind the Scenes

By Beacon Staff

Greg Trunkle hangs suspended in a cable car above the swollen brown North Fork of the Flathead River. Despite the water dropping below flood stage, it churns brown like a chocolate milkshake. For over an hour, Trunkle dangles above the rushing water to read the river, sliding a mere five feet at a time across its 200-foot width.

When the National Weather Service forecasts floods, its predictions are based in part on Trunkle’s work. He’s one of four streamgagers working for the U.S.G.S. in Kalispell. They each test two or three rivers per day from Missoula to the Canadian border and the Continental Divide to the Idaho state line.

Streamgagers have worked this North Fork site at Glacier Rim since 1910 – the first year readings were taken. The site’s automated readings taken at one static spot bounce water height through satellites every four hours, but streamgagers gather profiles of the shifting floor depth and velocity across the entire channel.

At some sites, streamgagers rely on technology designed in the 1930s – old-fashioned, hand-measuring methods for reading rivers. “It’s still really accurate,” says Lucky Sultz, a retired 27-year veteran streamgager. But recently, Doppler radar has simplified the job on about two-thirds of northwest Montana rivers.

At the North Fork site, Trunkle hauls gear out of his dark blue government truck. Sultz loads a 100-pound lead weight, shaped like a fat torpedo, onto a single-wheeled dolly – a tool the pair invented to ease the backbreaking haul. Like most in-the-field technicians, they fabricate what they need. This time of year, it’s a short walk to the shoreline; in winter, crews snowshoe to the site, lugging a sled and chipping holes in the ice to measure the stream flow.

Mid-river, Greg Trunkle releases the weighted streamgage back into the flow after reeling it up to remove tangled flood debris on the North Fork of the Flathead at Glacier Rim.

This is one of the “old fashioned” sites. Here, streamgagers take readings by hand. Trunkle climbs into a metal car hanging from the cable, sailing 20 feet above the water to the far shore, where he lowers the weight with a small splash into the murky torrent.

After grazing the weight on the river’s bed for a depth measurement, Trunkle reels it back up part way. Attached to the lead weight, a wheel with six tiny cone-shaped cups spins at the flow’s velocity and send an electrical click up the line. But in a surge this swift, the clicks come too fast. Adjusting the clicks – one sound for every fifth revolution – he counts the clicks for about a minute, converting the number to a feet-per-second flow.

Using a notebook dangling from a cord around his neck, he records the data and moves the car to the next mark on the cable. The car swings in the wind and bounces as the current pulls against the weight. “You can get queasy out there with the rushing water,” says Sultz, “but you’re concentrating hard on the readings, and you get used to it.” Trunkle eyes upstream too avoid snagging dangerous large floating debris and reels the weight up several times to clean small tangles off it.

As Trunkle swings above the river’s core where the current pulls the strongest, the cable hums. Sand and grit sweeping downstream ping against the weight – shooting a drone up the cable. On shore, the cable tower vibrates slightly. “When you’re out there over the water here, you hear a fizzing noise like Rice Krispies,” explains Sultz, attributing it to the gravel’s velocity.

At one point, Trunkle drops his pencil in the river and returns to the tower for a replacement. After an hour, he finishes his last reading near the cable tower.

Back at the office, he’ll transcribe all his hand-written data into the computer. “Once all this stuff went on the Internet, it got a lot more hectic,” says Sultz. “Everyone wants to know right now what the river is doing.” It’s not just the National Weather Service, river rafters, anglers, and irrigation managers, but the Flathead dams. Both Kerr and Hungry Horse dams rely on discharge data from streamgagers to regulate water releases.

Downstream, the Flathead River runs through Columbia Falls. Here, Trunkle and Sultz pull an orange trimaran from the truck. The small boat is equipped with a Doppler radar that reads the water depth and flow. Then, out comes a baby jogger, rigged to carry the computer and a bucket with the transmitter. “It takes longer to set up and test this than to get the measurements,” notes Trunkle.

Some waters are too rough for the Doppler system, and they’re expensive – around $25,000 per boat. But radars produce an instantaneous cross section picture of the river.

Trunkle pushes the baby jogger with one hand while holding the line to the boat with the other. In two round trips walking over and back on the Columbia Falls Bridge, he gathers readings on his computer. Total time? Fourteen minutes. “It would have taken two hours here the old way,” says Trunkle.

The streamgager data goes into the National Water Information Service for anyone to use. “We collect all the data for somebody else,” says Sultz. “Our job is just doing good science.”

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