May 19, 2011 - Bend Bulletin - A region at risk of floods
May 24, 2011
A region at risk of floods
As Eastern Oregon recovers, Deschutes Basin likely to withstand large runoffBy Kate Ramsayer / The Bulletin
Published: May 19. 2011 4:00AM PST
In Burns Wednesday, volunteers and crews were filling sandbags to help shore up a levee that stands between the Silvies River and part of the town, and the National Guard was on its way to help out.
Tuesday evening, road crews cut a trench through U.S. Highway 20 to prevent flooding of another area.
“The Silvies River has gotten out of its bank, it's made a new lake there that wasn't there last Sunday,” said Steve Grasty, a top Harney County official. “We have never seen the flows like this.”
In John Day, water from Canyon Creek that flooded the high school and many homes and businesses is receding, reports Grant County Sheriff Glenn Palmer. But officials are leaving the thousands of sandbags in place — in case the rain and snowmelt from higher-than-average snowpacks pulses back into town.
“We're in the recovery, but we're leaving things in place until we make sure that the water isn't going to come back up,” Palmer said.
But the Deschutes and Crooked rivers have avoided major flooding so far, although some stretches and tributaries are seeing higher than normal flows.
And water managers say the geology of the local river basins, along with lighter rainfall this week, means that Central Oregon will probably not face the flooding issues swamping parts of Eastern Oregon.
“Every little stream's a little different,” said Kyle Gorman, south central region manager with the Oregon Water Resources Department.
The Crooked River Basin, however, is like the John Day River Basin in that when it rains, the water can run off the fine grained, clay-like soils and cause flashy river flows, Gorman said.
“When it rains, it runs off almost immediately, or if the snow melts it runs off very rapidly,” he said. “The water doesn't infiltrate.”
The John Day Basin is much bigger, however, and there's more higher elevation areas where the snow can melt off — the Crooked River has a flow of around 3,000 cubic feet per second, Gorman said, while the John Day is about 10 times that much. And this last week saw more rain in the John Day area compared to in the Crooked River basin.
“Had we seen more precipitation, we would have seen a similar response,” he said.
In addition, although the flows of the Crooked River and Ochoco Creek picked up this week, there was space to store that extra water in the reservoirs on both waterways, said Mary Mellema, a hydrologist with the Bureau of Reclamation.
Reservoirs near capacity but unlikely to overflow
Prineville Reservoir is currently at 104 percent full — but that just means that water is flowing out of the spillway at Bowman Dam, like it was designed to, she said. And there's room for a lot more water, she added.
“The water's going to keep coming over that spillway, and eventually it'll just be such a little bit of water,” Mellema said.
Ochoco Reservoir is 97 percent full, but with the weather drying up a bit and the amount of water flowing into the reservoir dropping off, Mellema said it was unlikely to fill — and if it does, it will simply go over the spillway as well.
“We're thinking things are sitting pretty good,” she said. “We don't expect any major peaks (of water flow) coming from the snow.”
If the reservoirs remain full and weather forecasters start predicting major rainstorms, the Bureau of Reclamation can let more water out of the reservoirs, but currently that is an unlikely situation, she said.
Rob Tanner, hydrologist with the Deschutes and Ochoco national forests, saw that Ochoco Creek was full for almost seven weeks earlier this spring, and is back up again. Another rainstorm, especially in warm temperatures, could cause a lot of snow to melt and run into the river, creeks and tributaries.
“The biggest concern is if we get rain when it's really warm, or relatively warm, and the snow is melting,” he said. “We can get a big push.”
And that could cause flooding, Tanner said, if the Bureau of Reclamation lets more water out of the reservoirs because the storage areas have gone over their capacity — which is what happened during the floods of 1998.
Different geology in Deschutes Basin
In the Deschutes Basin, however, the geology of the area creates a different response to spring rains.
Rainfall and snowmelt seeps into the porous, volcanic soils in the basin and filters down to the aquifer, instead of flowing across the ground's surface to join up with a river. And the Deschutes River is fed by springs that provide a steady flow from that aquifer, which Tanner said has been compared to the size of the Great Salt Lake.
“It's a much slower response,” he said. “It takes a while for things to go through the ground and pop back out, there's less flashiness.”
But while the main Deschutes River is spring-fed, the Little Deschutes River gets its flow from both springs and run-off, said Gorman, with the Water Resources Department.
“We're keeping our eye on the Lower Deschutes,” he said. “It can, and has in the past, risen significantly. It's not fast, but it will rise.”
Flows on the Lower Deschutes are currently about 540 cubic feet per second, he said, and normal highs are between 400 and 500 cfs.
And, if a warm rain continues to melt snow, it could get up to 700 or 800 — or even 1,000 — cubic feet per second in the next 45 days, he said.
“We've seen 1,000 in the past,” Gorman said. “The river gets really wide at that point, it may come up onto people's properties in areas that they're not used to, but it's not a real rapid thing.”
Elsewhere in the Deschutes Basin, Tumalo Creek and Whychus Creek are also flashy creeks that can rise or fall based on rainfall and snowmelt — but spring isn't their problem season, Gorman said.
This time of year, the snowpack in those drainages is deep enough that the rain falling onto snow can be absorbed, he said, and then slowly melt off later. Floods from those creeks come in the late fall or early winter, he said, when rain hits light, fluffy snow and melts it all at once.
In Harney County, however, about 1.5 inches of rain fell over much of the drainage that feeds the Silvies River, Grasty said, and melted a higher-than-normal snowpack.
“It just started coming off,” he said.
Already, some houses are flooding and some residents are being warned to think about moving their things to prepare for a foot or so of water. If areas are flooded, it won't come in a fast wave, Grasty said, but will still cause damage. And it's not just a concern from the river, he said.
“The water table's raised so high, we've got a lot of little ponds and lakes scattered around town,” he said.
Due to the flooding, Highway 20 was closed at milepost 134 as of Wednesday evening.
In John Day, even though the waters have gone down considerably, businesses are still pumping basements and some county roads are still closed, Palmer said. Damage to the school alone is expected to be between $700,000 and $1 million, he said. There were also high flows off the north fork of the John Day River.
“I've never seen that river doing what it was doing,” he said.
Kate Ramsayer can be reached at 541-617-7811 or at firstname.lastname@example.org.
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