YANKTON, S.D. (KTIV) - Earlier this summer, more than a dozen communities in Northwest Iowa and Southeast South Dakota were flooded after heavy rain fell across the upper Midwest.

Flood waters rose from the Big and Little Sioux Rivers, the Rock River, the James River and the Vermillion River. Communities like the Spencer, Rock Valley, Hawarden and Cherokee in Iowa, and McCook Lake and Jefferson in South Dakota, were inundated with water within hours.

Still, months later, many are wondering how and why it happened.

“You look at the expanse of the Missouri River Basin covers nine states, and, 22% the whole geographic area the United States,” Michael “Moose” Welch is the Operations Project Manager with the U.S. Army Corps of Engineers at the Gavins Point Project. “Now you take that as a weather map, there’s a whole bunch of stuff that can happen and change and those are all dynamics, but they get magnified (at Gavins Point), because this is the last stop.”

The floods of 2024 brought widespread damage to Siouxland, but what made this flood different from others, like the Missouri River floods of 2011 and 2019?

“We try to react what we can, but sometimes we just can’t,” Welch said of the 2024 floods. Gavins Point is the last dam along the Missouri. The problem this summer, he says, is much of the water was flowing beyond the dam.

“The incident happened where all the rain was east of the Missouri River. Even Mitchell, 70 miles from the Missouri River, got eight inches rain,” explained Welch. “And then into northwest Iowa, and then the Sioux River, Big Sioux, Little Sioux, Rock River and James River, Vermillion River, they’re all east of Gavins Point Dam.”

Heavy rain north of Siouxland flooded tributaries and smaller rivers leaving all of that water with no place to go and downstream of Gavins Point Dam.

In a news conference in North Sioux City, South Dakota, on Sunday, June 23rd, officials expected the Big Sioux River to crest at 42.1 feet at 1 p.m. on June 24th.

In reality, it crested hours after that June 23rd news conference at 44.84 feet, the highest ever recorded, sending a wall of water into McCook Lake causing catastrophic damage.

“We are talking about historic flows on unregulated tributaries into the Missouri River,” said Col. Ron Newbauer of the U.S. Army Corps of Engineers at a June 24th news conference in North Sioux City, S.D. ahead of the McCook Lake-area flood. “It is hitting a lot of tributaries that flow unregulated into the Missouri so there is only a small part of hat that we can control.”

“This is where we start, we lose control of what happens,” explained Welch in August. “And again, in the story, on McCook Lake, even Mitchell, S.D., where it rained, is 70 miles east of where the river is in Chamberlain, South Dakota. So it was obviously a horrendous but hopefully unique event that happened at that time.”

The question from many this summer was, why didn’t the U.S. Army Corps of Engineers do more to stop the flood from happening? Releases from Fort Peck, the next dam above Gavins Point, were shut down and the lake above Gavins Point was full.

“In the situation with McCook Lake, we were already at 1210, which is our lake level normally,” said Welch. “But at 1210, that’s as much as the bucket holds, and then it starts coming over the top of the spillway gates. So it’s a dynamic moving equation all the time.”

Welch took KTIV to the top of the spillway where the lake meets the river to get a closer look.

“Right now we’re at 1207, so from the lake to the top of the gate is about three feet,” said Welch on a summer afternoon in August where all spillway gates were closed. “So when we look at the event in June, we were right at, like 1209.6, so we had about six inches left in the top of the gate. And so basically, we were full.”

From the floods of 2011, to today, to the future the Army Corps does what it can to prevent flooding, but there are a lot of factors out of their control.