Wednesday, December 7, 2011
At the mouth of the White Salmon River, a classic fan-shaped delta several acres in size is now clearly visible. And in true Gorge explorer spirit, as soon as enough land was above water this week, a kiteboarder tested the sand, set up gear, launched from the new landmass and explored the river around it.
The new sandbar was an anticipated aftereffect of the breaching of Condit Dam in late October, when nearly 100 years of sediment built up behind the dam was washed downstream in a matter of minutes. Although experts say its size, shape and permanency were uncertain, the fact that a delta has formed is really no surprise.
"We're keeping a close eye on it," said Tom Gauntt, PacifiCorp spokesman. "As you can imagine, the process is going to take some time and we'll see some changes. As winter continues and storms bring higher water and runoff, things are going to continue to evolve."
According to Corps of Engineers hydrosurveys, the new delta looks to be about as far from the shipping lane on the Washington side as the Hood River delta is on the Oregon side, so blocking large vessels on the Columbia isn't an issue. Gauntt said the Corps is certainly keeping an eye on the situation but, at this time dredging the delta isn't being planned.
An expert on hydrodynamics and delta formation, engineer Andrew Jansky of Flowing Solutions, spent considerable time studying and surveying the Hood River delta after the major debris flow in 2006.
"It's a natural process," he said this week after looking at photos of the new delta. "Sandbars are historically there. When dams were built the pool adjustment submerged them, and now they are naturally returning to pre-dam elevations. You can see sandbars reemerging at the mouths of tributaries up and down the Columbia."
Both Jansky and Gauntt noted that the delta will see changes, especially this year because high water and fast currents in both the Columbia and White Salmon haven't had a chance to shape the new landmass.
"It is going to continue to evolve, reshape and be distributed as the seasons progress," Guantt said. "We're expecting a lot of action through the winter due to high water. It's an unprecedented place, so it's difficult to say exactly what is going to happen."
As the surge of muddy water (flowing at about 10,000 cfs) from the Condit Dam breach met the slower pool of the Columbia, sediment quickly sank to form the delta. Looking at historical photos of the same area before the Columbia was dammed, a sandbar extended several hundred yards into the river. As with the Hood River delta, it existed historically for thousands of years, and now that it is back, the White Salmon delta, in some form, is more than likely around for good.
For those who are celebrating the emergence of a new beach and a solid new access point for on-the-water recreation, don't jump for joy just yet - there are several factors that will change the size and shape of the delta over time.
Unlike the Hood River sandbar, which is anchored by hundreds of thousands of cubic yards of large stone that washed down the river from the base of Eliot Glacier, the bulk of the new White Salmon delta is finer sediment that built up behind Condit Dam. Its makeup means it is much more susceptible to significant erosion and scour loss.
The delta is also on the upward side of a bend in the Columbia River, where the bulk of strong current is focused as it runs into the Washington shore. On the south side of the bend, the Hood River delta is in a large eddy and sees increased sediment deposits, whereas the White Salmon side of the river takes the brunt of the river's energy and will be susceptible to significant scour loss during seasonal high water in the Columbia.
Additionally, the new delta is at the end of a long, straight stretch of river to the west. The corridor is long enough for significant swell to form in summer months, when winds blow in the opposite direction as the current. The swell tends to roll in and break right where the new sandbar is situated, which will mean steady erosion from currents moving to the west and breaking waves moving east.
The White Salmon River can pack quite a punch during high-water events. At its highest point in recent years, flooding in 1996 saw about 40,000 cfs raging down the river. That's about four times the rate of the surge created from the Condit Dam Breach.
"The storm we had last week was the first real rain we've seen since the breaching," Gauntt said. "The thought is that when the river picks up, it's going to sweep a lot of that material farther into the Columbia, which will then carry it away."
Gauntt said PacifiCorp will complete a LIDAR study 90 days after the breach, which will provide a detailed view of the area's topography, showing land formation below the waters of both rivers.