I-29, which runs north from Kansas City, Mo., to Pembina, N.D., on the border with Canada and connects with Manitoba Highway 75, was blocked Thursday to build a temporary berm. As the river began dropping Friday, the berm was removed.
Floodwater did not reach the highway, allowing it to reopen immediately.
The 400-mile-long river forms the border between Iowa and South Dakota as it nears the Missouri. It also drains areas in southwestern Minnesota.
Earlier in the week, the National Weather Service predicted a crest of more than 109 feet, which would have set a record and could have brought devastating flooding. But the actual crest was about four feet lower, although still well above the 99-foot floodstage.
"We are extremely thankful that the flows were lower than forecast and residents of this area were spared major flooding," South Dakota Lt. Gov. Matt Michels said during a visit to the area Friday afternoon. "In these situations, you do all you can to prepare for the worst. Then you hope for the best. Many, many good people worked diligently to prepare for the worst. Fortunately, that didn't happen.''
Forecasters said the river would probably drop below flood stage on Sunday, assuming the area is spared heavy rain.