By MOLLY MONTAG
Sioux City Journal
BURBANK, S.D. -- The half-million-dollar nightmare that threatens to eat away Bob Larson's property, sweep away his home and siphon the bank accounts of the entire riverfront cabin community he lives in didn't have an ominous beginning.
Churning quietly underneath the surface of the Missouri River, it ground up the sandy riverbank and river bottom. It swirled away behind and below the line of boulders Larson and his neighbors bought to fortify the riverbank, carving out the sand underneath the rock.
It was an eddy. And, eventually, it won.
"All of the sudden, it just started caving in behind the rock," Larson said earlier this month.
Big trees went down. Tall, fully grown cottonwoods as big around as a man's arm span crashed into the watery pit, he said.
Crews laid more rock. That barrier held for a few weeks. Larson noticed something was amiss when he went outside one morning.
"I noticed my furniture was gone," he said, recalling when the saturated sand gave way. "My lawn furniture is down in the trees."
State and federal officials say the forces that are threatening Larson's home are creating changes in the river: cutting deep holes or gouges in some places, dumping large sand deposits in others and even changing the course of the river itself.
Steve Mietz, superintendent of the Missouri National Recreational River for the National Park Service, said the flood accelerated the river's natural process of eroding some areas and rebuilding others. At the height of the flood, the U.S. Army Corps of Engineers ordered 160,000 cubic feet of water per second released from Gavins Point Dam near Yankton, S.D.
"It had a lot more energy and a lot more water," said Mietz.
On Monday, the corps plans to begin daily 5,000-cfs reductions in the flows from the dam, which were recently reduced to 150,000 cfs. The restricted releases will continue until Aug. 27, when the discharge reaches 90,000 cfs.
Releases will be held at 90,000 cfs for approximately two weeks, and then the flow will be reduced by 5,000 cfs every two days until Gavins Point Dam is releasing 40,000 cfs. The corps expects the flows to hit that mark on Sept. 30.
CHANGING RIVERSCAPE
Across the water from Larson's battle with the current, a mobile home looked likely to lose a battle of its own. It was poised in mid-nose dive off the edge of the forested shoreline, one end submerged up to its window with the opposite end tipped up several feet in the air.
In addition to once being on solid ground, the trailer used to be on the mainland on the Nebraska side of the river. With surrounding farm fields flooded, the trailer looked ready to slide off the edge of an island.
Experts say the water is at work in fields like those, carving deep holes in some while dumping trash, debris and sediment in others. The river may even be attempting to cut new channels in parts of southwest Iowa and northwest Missouri affected by levee breaches, they said.
John Remus, chief hydrologic engineer for the engineering branch of the corps' Omaha District, said the river doesn't appear to be cutting new channels in Siouxland, but receding waters may expose deep holes along the stretch between Sioux City and Ponca, Neb.
"You're probably going to see a little more damage in the flood plain as far as scour holes," Remus said. "You may see some very long scour holes that would appear to be the beginning of a channel forming.
Some of those holes could be 25, 35 or 40 feet deep, Remus said. It would be a similar phenomenon to 1993, he said, when the river carved 50- to 60-foot-deep holes south of Omaha that still hold several feet of water.
"It would be basically scoured down to the base of the river and could hold water for years," Remus said.
Even if the river doesn't cut a new channel, it will change. The main channel will likely shift around islands and sandbars, Remus said.
"I would be very surprised if we don't have a shift of the main channel from one side of the river to the other in a number of locations," Remus said.
There also is erosion of the riverbanks, but Remus said the corps hasn't closely monitored the amount of bank erosion.
Officials in Sioux City also won't know how much of the city's riverfront has eroded until the water recedes, but city Public Works Director Chris Payer said crews have already identified some potential trouble spots. One of those is the walking trail through Chris Larsen Park.
"That was showing evidence of being undermined even before we closed Larsen Park Road," Payer said.
WATER LEVEL COULD DROP
The massive rush of water is also ripping up the bottom of the river, something South Sioux City City Administrator Lance Hedquist believes could result in a 6- to 8-foot drop of the riverbed at the Nebraska city.
At the height of the flooding, Remus said, officials believe the force of the water eroded 7 to 8 feet of the river bottom in Sioux City, less near Blair, Neb., and 5 to 6 feet near Omaha. The riverbed will rise again to some degree as the flows decrease, Remus said.
"As the flood recedes and the sediment is no longer scoured but drops out," he said.
The stretch of river from Gavins Point Dam down past Sioux City is called a degredational region, which means the river bottom degrades more than it replenishes itself over time, Remus said.
It's unclear whether the flood will change the long-term degradation rates in the region, but so far officials don't see an increase in the degradational trend.
If the water does not replenish the riverbed, Hedquist said, lower water levels could have a dramatic impact on the South Sioux City shoreline, a riverfront lagoon and Crystal Cove Lake.
He said officials will likely need to stabilize stormwater drains that may be exposed when the floodwaters recede and decide whether to dredge or fill in a shallow lagoon in Scenic Park. If the river level drops as much as officials believe it might, Hedquist said the water line will be lower than the 5- to 6-foot-deep lagoon people often fish out of at the park.
Similarly, although parts of Crystal Cove Lake are 45 feet deep, Hedquist said a significant drop in the river level and water tables could expose shoreline and require dredging at a cost of more than $1 million.
"The water level, if it drops by 7 feet, would greatly reduce that area for recreational activities," he said.
Also of concern are two pipes underneath the river. The pipes, which carry wastewater from South Sioux City to Sioux City for treatment, were buried 12 feet under the bed of the river. Officials will check to make sure the river hasn't eroded down to the level of the pipes, Hedquist said.
TREES, WILDLIFE ALSO AFFECTED
Officials say the high water isn't affecting only the river's flow and the people who live along its banks. It's also changing habitat for wildlife that live along the river. For some animals, that's a good thing. Others, particularly those that prefer to live in or around trees, won't be so lucky.
Carl Priebe, a wildlife biologist at the Nishnabotna Wildlife Unit in southwest Iowa, said experts believe 90 percent or more of the trees soaked by floodwaters will drown and die. Trees need air in their roots, he said, which they haven't been able to get after months in the water.
"Some of them might show stress for a year or two, but within two to three years we expect a majority of them to be dead," he said.
City officials expect a similar tree die-off in South Sioux City, where 800 to 1,000 trees in Scenic Park will either die or be cut down for safety reasons.
"We can't afford to have a cottonwood tree that's 75 feet tall fall," Hedquist said. "You have people living there, so the reality is virtually every one of the very large old cottonwood trees are either going to go by themselves or are going to have to be taken out of there."
Sioux City Public Works Director Chris Payer said officials have already observed dying trees in Chris Larsen Park.
The death of so many trees will likely have a negative impact on songbirds and deer, which will also be affected by crop loss along the river, Priebe said.
"If the trees are dead and there's not much cover and there's not much food for (the deer) to eat either, they might not return as quickly as they could," he said.
On the other hand, Priebe said the tree loss could be a positive change for woodpeckers, bats and other animals that thrive on dead-tree habitat.
Experts say the flooding could also increase habitat for fish and shorebirds.
Although the waters will have washed away sandbars built for piping plover and least terns, Mietz said the flood will create new islands or sandbars for the threatened and endangered birds to nest.
"We're expecting many more islands will be built," he said.
The slow-moving waters are also beneficial to fish, which use the areas as natural hatcheries, Mietz said.
WAIT AND SEE
Although biologists believe they know some of the effects the flood will have, Priebe said everyone will have much to learn when the floodwaters recede.
"A lot of that we're going to have to learn because we haven't seen these kind of floods in Iowa or the Midwest," he said. "Typically we see floods that last two or three weeks and they go down and things get back to normal."
Terry Brady, of rural Burbank, S.D., said the river's changed dramatically in the approximately 20 years he's lived alongside it. He moved there during a drought, when a person had to walk 150 yards into the riverbed before hitting water.
Now, Brady, Larson and other residents of the Ponderosa development have spent nearly $500,000 laying 3,600 feet of rock to keep the riverfront from overtaking their homes. They hope it will hold, but Brady said the river sets its own course.
"It all depends on Old Mo, where she wants to channel," Brady said.
