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A lone fisherman rode his bike to get out to a sand bar in the middle of the Cowlitz River near Longview last week. Greg Ebersole / The Daily News

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Home > Top Story

Cowlitz flood risk growing, study reveals

Monday, August 4, 2008 12:32 AM PDT

By Andre Stepankowsky

Printable version E-mail this article 30 comment(s)

A new federal study finds that the Cowlitz River is "significantly" more prone to flooding than previously thought and that Castle Rock may no longer be safe from a 100-year-flood.

The study by the U.S. Army Corps of Engineers may trigger an effort as soon as this winter to beef up the Castle Rock levee. Longer term measures to address the threat are at least several years off, however, the agency says.

The study found that Lexington, Kelso and Longview levees still can withstand 100-year floods — which have a 1 percent chance of occurring in any given year — and likely even larger ones. Even so, flooding odds are increasing along this part of the river, too.

The buildup of channel-clogging volcanic silt in the river is only part of the reason. The biggest reason, the corps says, is that climate change, or at least a cycle of wetter weather, has made the basin more prone to major storms over the past two or three decades. Mossyrock Dam, one of the principal flood control works on the Cowlitz, cannot prevent or blunt all major Cowlitz floods, the corps says.

Corps officials told The Daily News last week that they have begun assessing dikes along the lower Cowlitz to see if it is possible to strengthen weak points.

A plan is due in December.

Dikes wouldn't need to be raised. But in places they may need to be widened or made less prone to seepage. It's possible, for instance, to bore into a levee and fill it with concrete, according to the corps.

This week, the corps will start dredging a mile-long stretch of the river upriver of Gerhart Gardens Park in Longview. Under a $1.97 million contract, Ross Island Sand & Gravel of Portland will remove some large shoals built up in the river by the flow of volcanic silt from the Toutle River.

The dredging, planned more than a year ago, won't do anything to reduce the risks up at Castle Rock, said Tim Kuhn, the Cowlitz-Toutle project manager for the corps' Portland District. The corps will not dredge the Cowlitz River near Castle Rock any time before 2010 because it has neither the money nor environmental clearance, Kuhn said.

That news leaves Castle Rock Councilman Greg Marcil angry and scratching his head, wondering why the corps is dredging the lower river, which still has adequate flood protection, and not the reach near Castle Rock.

"Why don't we dredge the river to lower the bed?" he asked.

"Everyone is going to have to start writing their senators. We need something done. ... I would hate to have wait until the town is wiped out and then they do something."

Castle Rock public works director Dave Vorse and Cowlitz County Commissioner Axel Swanson said they were disturbed by the news after a briefing by the corps two weeks ago, but they said there's no reason to panic.

"I wouldn't saying it was alarming, but it wasn't good news," Swanson said Friday.

Vorse pointed out that Castle Rock's dike withstood three major floods in the last dozen years - in November 1995, February 1996 and November 2006.

However, the corps concluded that floods of that size or greater will occur more often than previously thought.

"Bigger floods are more frequent," said Patrick O'Brien, a corps hydraulics engineer and one of the principal researchers of the study.

For example, the annual odds of having another flood like the one in February 1996 have increased from 1 in 160 to 1 in 80.

Plus, when a 100-year flood hits, it will be about 20 percent bigger than in the past, the corps estimates. In Castle Rock, that means the river would be a foot higher and rise to the very limit of what the levee can withstand, according to the corps. The town would have a one-in-five chance of flooding.

The study assessed the condition of the entire Cowlitz Basin, including the impact of Mount St. Helens' eruption and the history of the drainage's climate and river flows. The U.S. Geological Survey, U.S. Board of Reclamation and Corps' Hydrologic Engineering Center in Davis, Calif., helped the Portland District do the study.

Hydrologists have noted that other Southwest Washington rivers have had bigger floods in the last few decades, reflecting climate shifts, O'Brien said.

In one sense, the Cowlitz analysis is just catching up with the commonsense observation many astute river watchers have made in recent years: If the chances of having a 100-year storm are 1 percent in any given year, how come so many occurred in just one decade?

Another reason flooding odds have increased has to do with Mossyrock Dam. The horseshoe-shaped concrete dam rises 606 feet above the riverbed and holds back a 12,000-acre Riffe Lake. Many times, the dam has held back water and prevented or weakened floods on the lower river.

But it doesn't control the entire basin. For example, the dam doesn't have any control of the Toutle River, a major contributor to the February 1996 flood. In addition, it's full flood storage space is not always available. The November 1995 flood occurred before the reservoir was fully lowered to handle major storms.

The odds of flooding also are rising on the Cowlitz River with every cubic yard of debris the Toutle washes into it from Mount St. Helens. In the Castle Rock area in particular, heavy, coarse-grained material is difficult for the Cowlitz to flush away, according to the corps.

In 1985, the Corps took responsibility to maintain flood-protection levels on the lower Cowlitz River, and Congress affirmed that in 2000.

In addition to developing a plan to bolster dikes, the corps is developing a long-term strategy to cope with the flow of volcanic silt out of the river. That plan is now scheduled for completion late in 2009 — about a year later than originally scheduled.

In the mid 1980s, the corps built a dam on the north fork of the Toutle to trap the silt, but the structure quickly filled up and is passing debris downstream. In fact, so much silt is now clogging the lower Toutle that the river now shifts wildly during storms, causing it to cut its banks and eat away dredge material pulled out of the river in the 1980s.

So the corps needs a new long-term plan and is dredging the Cowlitz periodically as a stopgap measure. That's the reason for this week's dredging above Gerhart Gardens.

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