Minor flooding in the area is already occurring. Some residential roads are covered with standing water.
"This is one of the highest years I've seen in the fall, ever," said John Hausz, who lives in Ft. Atkinson.
But the National Weather Service expects the river to continue rising to nearly 11.8 feet by Monday afternoon.
"If the water keeps coming up, we have to start sandbagging and all that other fun stuff," Hausz said. "Right now everybody's in wait-and-see mode."
Should the river reach 12 feet, the National Weather Service said there will be "widespread flooding of lakeside homes and roads in the Lake Koshkonong area including Ralph Road, Vets Lane, Blackhawk Island Road, and Oxbow Bend Road."
Patten's Marine, which is located along the river on Highway 106, has already lost money because of the high water.
Wes Johnson, the businesses' owner, said the shop has a relatively short window each year to make money on boat sales and repairs.
But he said many customers have already pulled their boats from the water this season, earlier than usual, because of the rising Rock River.