SIOUX CITY -- For hundreds, thousands of years, the Missouri River flowed as it pleased without resistance.
For most of the year, it meandered lazily within its wide, shallow riverbed, its ever-changing currents periodically rearranging sandbars. Twice a year, it swelled, filled its channels and, at times, spilled over its banks into the broad adjacent flood plain.
Once settlers pushed west, they began efforts to tame the river so as to tap its economic, recreational and aesthetic potential. They built dams, levees and other structures aimed at keeping the river in check so that businesses and homes could be near the river, taking advantage of all it had to offer.
But despite man's best efforts, the Missouri River, as it is demonstrating now, will do whatever it wishes.
"The Missouri River is a large, powerful river, and it's showing its strength this year," said Tim Cowman, director of the University of South Dakota's Missouri River Institute. "No matter how much we try to manage and engineer the Missouri River, there are going to be years the Missouri River has its way with us.
A flood the scale of 2011 -- and other historic floods in 1881, 1943 and 1952 -- needed certain natural forces to line up perfectly. Cowman said lots of melting snow and above-average spring rainfalls in each case combined to fill a river basin that drains one-sixth of the United States with more water than it can handle.
Mother Nature isn't the only culprit, experts say. Decades of human actions designed to curb and prevent flooding have contributed to the present flood's severity.
"For far too long, we've considered our rivers as problems that need to be solved," said Shana Udvardy, director of flood management policy at American Rivers, a nonprofit conservation organization based in Washington, D.C. "We've walled off and dammed up the Missouri River, and we're seeing a lot of consequences downstream."
This will be a flood of historic significance, not only because of the amount of land it ultimately covers and property it damages, but because it is the first of its kind to take place since the public was assured that major floods were a thing of the past.
HISTORY OF CHANGE
The flood of 2011 comes as no big surprise to those who study the Missouri River and know its annual rhythms. It rises in April, when snow from the Plains melts and fills the river and its tributaries. In June, it rises again when melting snow in the mountains sends a second pulse of water downstream.
This year, as in other years of significant flooding, those natural forces aligned just right. Mountain snowpack is above average, in some places more than 180 percent of normal, according to the Natural Resources Conservation Service's National Water and Climate Center. At the same time the mountain snow began to melt, record amounts of rain fell in parts of the drainage basin, sending even more water into the Missouri.
Those conditions hadn't aligned at this level since 1952, before the series of six dams on the Missouri River was completed. Unfortunately, memories of that event faded and led to a false sense of security that was furthered by the presence of the dams and other flood control structures.
"When we live along the river and go for decades of not seeing flooding, we get used to it, and that's when we build structures and infrastructure next to it," Cowman said. "Then we get a year like this."
To better understand the current predicament, one must go back several decades, said Robert Schneiders, a Sioux City native, environmental historian and author of "Unruly River: Two Centuries of Change Along the Missouri," a book that examines the U.S. Army Corps of Engineers' management of the river.
In the 1930s and 1940s, the corps began channelizing the Missouri in order to make navigation easier for boats and barges that floated goods up and down the river. At roughly the same time, the corps began the dam-building.
A project of that scope required hundreds of millions of dollars of federal appropriations. To secure the funding from Congress, the corps had to make a strong case for the project. So, the corps told legislators that the floods would stop with the building of the dams, said Schneiders, director of Eco InTheKnow: Environmental Consultancy in Boulder, Colo.
"It certainly curtailed those floods," Schneiders said of the system of dams stretching from eastern Montana to southeast South Dakota. "They have achieved a certain level of flood prevention."
But downstream, there now was a new set of circumstances.
Some 8,300 control structures such as wing dams and revetments have been built in the river from Ponca, Neb., to the mouth of the river in Missouri. Their purpose: direct the channel toward the middle of the river and keep the river inside its banks.
That deeper channel ensured that boats and barges could better navigate a historically challenging river known for switching channels day to day. It cut down on flooding and left huge areas of the floodplain dry so farmers could till its fertile soil.
It also resulted in a deeper, narrower, faster river. Prior to channelization, Schneiders said, the Missouri River was 5,000 to 10,000 feet wide at Sioux City, going by at about 2 mph. Now, it's 740 feet wide, traveling at 6 mph.
Before the levees and other structures, when the June rise occurred, the river could spread out. The slower current carried less energy, less destructive force.
Now, the river can only go higher and faster when there's more water in it.
"(Channelization) has lowered the river's capacity to carry floods," Schneiders said.
DAMS PROLONG PULSE
The reservoir system, while capably controlling flooding during years of normal snow and rainfall, also will play a role in the severity of this year's flooding. They hold back all that extra water, saving thousands of acres of land from being under water every year.
The dams, however, have changed the nature of the flooding, Cowman said. In predam days, that June surge of mountain snowmelt would come down the river in one large pulse and flood the river valley. Once the pulse was past, the river returned to its banks.
That pulse is now slowed by the reservoirs. The corps can control it by releasing water through its dams. This year's extreme conditions have led to record reservoir releases, which will continue for weeks. As a result, water will remain high for a longer period of time and flood waters won't recede as quickly.
And now, there's more valuable property -- city infrastructure, industrial plants, expensive homes and recreation areas -- next to the river. There certainly will be more property loss during this flood than in 1952 or previous floods.
It's easy to call the decision to build a house or business near the river foolish, Schneiders said, but those who did so believed the premise that engineering had eliminated the threat of flooding, even in areas of sandy river deposits that resulted from floods of years past.
"Because people are lulled into believing floods will never come, people began building right up to the river. I think the building of Dakota Dunes symbolizes that," he said. "I think, again, they're putting their faith in the Corps of Engineers and dams in South Dakota to protect their property."
As the Missouri River has flooded in recent days, frustrated property owners have criticized the corps for its management of the river. Many want to know why the corps didn't begin releasing water from reservoirs earlier when it knew there was above-average snowfall in the mountains.
Col. Bob Ruch, commander of the Corps of Engineers Omaha Division, said the corps follows a master manual to set reservoir releases during the fall and winter so that once spring arrives and the snow melts, there is enough room to hold the influx of water.
Utilizing all its data, the corps had released sufficient amounts of water in advance of the spring snowmelt, Ruch said.
"We were properly prepared for it," he said.
What the corps couldn't prepare for, Ruch said, was a two-week period in May in which two heavy rains dumped a year's worth of precipitation on parts of Montana, Wyoming and the Dakotas. All that rainfall rushed into the Missouri River and its tributaries and began filling the reservoirs quickly, just as water from melting snow was making its way in, too.
"We just couldn't keep up with the inflow," Ruch said.
In May, 10.5 million acre-feet of water flowed into the system above Sioux City, Ruch said, shattering the previous May record of 7.2 million acre-feet, set in 1995. Overall, May 2011 is the second-highest month for inflows since records were first kept in 1888.
Increasing reservoir releases earlier likely would not have helped the current situation, Ruch said, because even now, water is entering the system as fast as it can be discharged. The record-breaking releases would still be necessary.
"Once that water came into the system, we are now reacting to it," he said.
Cowman said it's hard to blame the flood on the corps.
"Nobody anticipated that we would have these kinds of rainfall events," he said. "It was a weather event that was unforeseen and unpredictable."
Sometimes, the corps doesn't make the correct decision, Schneiders said. And sometimes, it isn't able to.
While managing a 2,351-mile-long river, the corps must pacify a broad range of interest groups. The interests of conservation and environmental groups often are at odds with what real estate and business developers want. There are recreational interests to please. Barge shippers have a different set of needs.
"All the interests along the river, the corps is trying to coordinate their reservoir releases and trying to placate all these interests," Schneiders said. "That's a hard juggling act."
Ruch acknowledged that wide array of interests. The purpose of the river system is not just flood control, but also hydroelectric power generation, irrigation, navigation, ensuring water quality to municipalities, recreation and environmental stewardship.
"It's managed according to the master manual. The master manual takes into account all users," Ruch said.
QUESTIONS FOR THE CORPS
Whenever the flood waters recede, fingers will be pointed, many of them toward the corps.
"Looking at this flood, I think the corps' role both historically and in the present needs to be assessed," Schneiders said. "A certain level of questioning is appropriate for levels of operation for the Army. The corps doesn't always get it right."
Ruch said the corps continually reviews its master manual, especially after a major event such as this. He's heard many of the accusations that reservoir releases were held back for the benefit of others. He said releases this spring were not managed with endangered birds in mind, nor were they held back so as to keep lakes high for boaters.
"I have no doubt we'll have plenty of questions to answer," he said. "I'm confident we acted according to the master manual. We believe we're in the parameters of the master manual.
"What's amazing to me is that the system has operated as well as it has. Without this system and the dams in place, inundation to this basin would be catastrophic."
Regardless of any river management plan, one fact remains: the Mighty Mo, when fully revved up is impossible to stop, no matter how many dams the U.S. Army Corps of Engineers builds or how much water is released from its reservoirs.
History shows that the flood waters will recede, the river will return to its banks. Lessons will have been learned. River management practices will be reviewed, maybe even altered.
But at some point, the Missouri River will again remind those who live near it that it's in charge. It sent that reminder in 1881, 1943, 1952 and, now, in 2011.
"This year isn't the last year," Cowman said. "It's going to happen again."