The dangers posed by the buildup of rocks and boulders in riverbeds at Mount Rainier National Park is best seen on the park’s northern edge.
The Carbon River valley is so choked with rocks that floodwaters in 2006 found it easier to race down the roadway, washing away large swaths of pavement.
Along the White River, state Route 410 flooded in 2003 and 2005 because the riverbed in some spots is 16 feet higher than the road.
The issue of aggradation – how fast streambeds fill with rock and sediment – first came to prominence along these two rivers.
Park staff members found the aggradation rate in the park’s rivers and streams has risen dramatically. From 1910 to 2000, the rate was 4 inches a decade. In the last 10 years, it has skyrocketed to 3 feet a decade.
“We’re literally choking on sediment and rocks,” said Paul Kennard, the park’s fluvial geomorphologist.
Aggradation is a major issue on rivers with braided channels – such as the White and Carbon – where the river channel is pushed from one side of the bed to the other as rocks build up and creates several smaller side channels.
“Braided rivers have more sediment than they can move,” said Tim Abbe, a geomorphology expert with Cardno Entrix, the Seattle-based firm that has done consulting work for the park on this issue.
The Carbon River is a good example of the problems caused by aggradation.
“In the ’90s, I can remember saying, ‘I’m glad (the river) is on the north side of the valley,’ ” said park superintendent Dave Uberuaga. “But it was aggrading and aggrading, and then it fell off the table and came over to the south side.”
Then, when nearly 18 inches of rain fell in 36 hours in early November 2006, the river flooded. Because water seeks the easiest path, the rising river moved farther south through the forest and roared down the road from Ipsut Creek Campground toward the park entrance.
“An old-growth forest floor is quite rough, offering a lot of resistance,” Kennard said. “That’s why the water opted to flow down the smooth roadway.”
The road has been converted to a hiking-biking path since the flood.
In an environmental assessment released in September, park staff members recommended reopening the road to vehicles 1.2 miles from the entrance to the turnaround at the Old Mine Trailhead. From there, the road would be converted into an improved trail.
The estimated cost is $3.2 million. A decision is expected early in 2011.
On the White River, options include rerouting the road to higher ground or raising the level of the road, said Eric Walkinshaw, the park’s civil engineer.
MAN, NATURE WORKING TOGETHER
To slow river flows and protect banks, the park has installed rock barbs along both rivers. Uberuaga also hopes to find the money to install a series of engineered logjams along the Carbon River.
Working in the park’s favor, at least along state Route 410, is the health of the old-growth forest, Abbe said. Some trees standing between the river and road are more than 500 years old. The older, larger trees slow the force of flooding rivers.
“Young trees don’t have as much resistance and are more likely to get blown out than older trees,” Abbe said.
“It’s clear that without that forest, we’d lose Highway 410,” he added.
The concern is when rocks and sediment left behind by floods and debris flows build up in the forest. That causes the trees to die and kills plants along the forest floor, reducing the forest’s capacity to slow floodwaters.
That’s happening along Tahoma Creek on the park’s west side. About 200 acres of old-growth forest has been lost due to debris flows, said Scott Beason, park geologist.
On a hike along the creek, one can see a ghost forest – dead snags rise 50, 60, 70 feet into the air.
“What’s happening on Tahoma Creek is mostly suffocation,” said Lou Whiteaker, plant ecologist at the park. “Even these big conifers, the big root mass is within the top 12 inches of soil. If you cover that with three feet of sediment, the roots aren’t getting any oxygen.”
Without the forest to slow the water and a streambed filled with rocks, the stream meanders across the valley floor until it finds a low spot.
Beason pointed out a spot on Tahoma Creek where floodwaters, pushed toward the west bank because of the build of rocks in the main channel, easily poured into the forest.
Not 100 feet into the forest, the water found an old channel filled in by dirt and rocks over the years. The force of the water had carved a new channel more than 6 feet deep in spots. Rocks the size of basketballs were strewn across the forest floor, crushing small bushes and low-growing plants.
“The power of water to sculpt the land is just amazing,” Beason said.
Jeffrey P. Mayor: 253-597-8640 Jeff.mayor @thenewstribune.com