When the rain forecast to hit Johnson County late Tuesday curved south instead, it spared the soon-to-be brimming Coralville Lake and possibly prolonged expected flooding downstream.
“That was a big deal for us right now,” said Larry Weber, director of IIHR – Hydroscience & Engineering, the parent organization of the Iowa Flood Center. “If it had moved 75 miles to the north, we would be in a difference situation.”
Still, every drop of rain that falls anywhere along the sprawling Iowa River Basin – one of the state’s major basins spanning 3,100 square mile – affects management of the outflow at Coralville Lake, Weber said.
Heavy rain along the northern portions of the Iowa River, for example, can mean an increased outflow from the lake – making more room for the incoming water. Precipitation into the lake itself could result in a limited outflow as officials try to let the new water pass out of the system, Weber said.
Rain downstream also can mean a decreased outflow so communities in those areas can manage the water they already have, he said.
The National Weather Service forecasts plenty of storms in the coming days – Iowa City specifically is expected to experience heavy showers Thursday and Friday.
“But it’s hard for anyone to forecast where this rainfall is going to land,” Weber said.
'At a tipping point'
The Iowa River Basin starts in north central Iowa near Garner – west of Mason City – and it flows down through communities like Belmond, Iowa Falls, Marshalltown, Tama, Marengo and then Coralville and Iowa City. Its primary supplier is the Iowa River, but it also draws in smaller tributaries in the form of creeks and streams, Weber said.
The Iowa River and Cedar River basins run parallel, and they join near Columbus Junction, where they become the Iowa River and then feed into the Mississippi River.
Coralville Lake, which sits near the end of the Iowa River Basin, was built in the late 1950s for both flood protection and recreation with a spillway designed to hold up to 712 feet of above sea level. It has flooded twice – 1993 and 2008.
Weber said the lake can handle a lot of rain in a short period of time as long as it is followed by a dry spell that allows the water to flow out of the basin and out of Iowa. Active weather patterns and long periods of rainy weather, however, can overwhelm the system.
“And if we get more rainfall in the next 48 hours, we likely will see it come over the spillway, and the Coralville Reservoir will go into an uncontrolled release,” Weber said. “We are at a tipping point right now.”
Several spots along the Iowa River, upstream of the Coralville Lake, had reached flood stage as of Wednesday including Marshalltown at 19.84 feet, Tama at 21.62 feet and Marengo at 22.32 feet, according to the U.S. Army Corps of Engineers.
Downstream, Lone Tree was over flood stage at 17.42 feet, as was Columbus Junction at 22.09 feet and Wapello at 24.57, according to the corps.
To balance the flooding upstream with the flooding downstream while bracing for more precipitation, the corps on Wednesday increased the amount of water coming out of the lake from 6,000 cubic feet per second to 10,000 cubic feet per second, according to Rick Fosse, Iowa City’s public works director.
Officials initially had hoped that level would hold for two weeks, but now Fosse said the corps expects to increase the outflow to 12,000 cfs as soon as Thursday and possibly go to 14,000 cfs on Friday.
Dubuque Street in Iowa City, near City Park, starts to flood at 12,000 to 13,000 cfs, according to Fosse, who said he expects the risk of flooding to threaten for “a number of weeks.”
“Unless we settle into a dry period, this is not going away quickly,” he said.
Time to make preparations
Flood events can sometimes go on for weeks or longer because of the time it takes for water to flow downstream and because of the management process that occurs at the Coralville Lake, said Dee Goldman, operations manager at Coralville Lake for the Corps of Engineers Rock Island District.
“We are doing a balancing act,” Goldman said. “How much do we discharge into the downstream communities while trying to evacuate water in preparation for more rainfall?”
As rain continues to drive up river levels at locations upstream, Goldman said, the corps meets with community leaders downstream to make them aware of the risk that water will pour over the spillway. They discuss how much water can be discharged to avoid that, Goldman said.
“Some of the downstream areas needed time to make preparations for more water,” he said. “So we gave them a heads up and let them prepare, and now they are ready.”
Those downstream communities and entities in this case include Coralville, Iowa City, the University of Iowa and Hills. The lake is expected to hit its spillway crest at just over 712 feet – its capacity – on June 7.
But Goldman said a lot of the flood forecast depends on where the rain actually falls. And that crest could come sooner, or it could be scaled back.
“At this stage, rain wouldn’t be ideal anywhere,” he said. “We need a period of time where we don’t have water coming in so we can evacuate the water and get rid of it."