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Will warm weather bust up ice jams?

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Catherine W. Idzerda
January 25, 2008

First the good news: Higher temperatures Saturday mean ice jams along the Rock River might start to break up, according to the National Weather Service in Sullivan.


Now the bad news: Flood warnings for the area between Highway 14 and the state line have been extended until 4 a.m. Sunday.


For residents who live along the Rock River between Janesville and Beloit, neither piece of news answers the question foremost in their minds: When will the icy waters around—and in—my home recede?


That’s hard to say.


“That darn ice is so unpredictable,” said Shirley Connors, Rock County emergency management director.


As of 1:15 p.m. Thursday, the water level readings at Afton were at 11.62 feet. Flood stage is 9 feet. The weather service predicts water levels will peak at 12.5 feet at about 11 a.m. today, according to a news release from the Rock County Sheriff’s Department.


“That’s just a prediction,” Connors stressed.


However, flood levels are going up in Rockton, Ill., which seems to indicate that some of the water is getting through, Connors said.


South River Road remains closed from Highway 11 to Happy Hollow Road. Oakley Road, the entrance road to Happy Hollow Park and Christianson Lane also are closed.


“It is highly treacherous for motorists to travel on these closed roadways,” the news release said.


Free sand and sandbags are available to river residents around the clock at the Rock Town Hall on Afton Road. Sand and sandbags also are available at the Rock County Airport entrance off Oakhill Road near the maintenance building. Residents will need to bring own shovels.


About 10 area families have evacuated their homes, and four families are receiving assistance from the Red Cross.


So far, the Indianford and Newville areas have not experienced flooding, nor have homes in the city of Janesville.


“We just keep watching and waiting,” Connors said.


Understanding the impact, dangers of an ice jam

Q: What is an ice jam?


A: It’s a dam of frozen, tumbled ice that limits the flow of water in the river.


As chunks of ice race down the river, they get caught on a tree or bridge. The water forces the chunks on their sides, which blocks the water.


More ice crashes into the first deposit, and the pieces freeze together until they resemble a giant, frozen beaver dam.


Q: Ice jams are rare around here. What makes them happen?


A: Rock County got more snow and cold weather than usual early this winter, followed by a fast melt, said Herb Garn, hydrologist with the United States Geological Survey. According to The Janesville Gazette’s records, 39 inches of snow has fallen this season in Janesville compared to 11 inches at this time last year.


Then in mid-January, the snow suddenly melted and an inch of rain fell. That broke up thick layers on lakes and rivers, sending ice chunks careening downstream.


Eventually they catch on banks, trees or bridges, and that’s when the trouble starts, Garn said.


Q: What’s the record level on the Rock.


A: The record level was 13 feet, caused by an ice jam in 1916, Garn said.


Q: Why can’t you just blow up the jam?


A: Ice is too flexible, Garn said.


“You blow it up, and it piles back up again and fills with water,” Garn said. “It’s not an easy solution.”


Q: What’s going to happen when the ice melts?


A: Nobody knows. It depends on how quickly it warms up and how much water comes down the river from the north.


The best scenario calls for the river to slow down and stop forcing more ice into the jam, Garn said. Then the jam could break up slowly.


But if it suddenly warmed up, allowing ice to move, and then froze again, we could be in the same boat, Garn said.


The USGS has increased the frequency of readings on its Afton gauge to get results every two hours, Garn said. That way, it has useful data for residents closely watching the river.


Q: How does the National Weather Service predict when and where the river will crest?


A: It’s better than a crystal ball. But it’s not an exact science, said meteorologist Chris Kuhlman with the National Weather Service in Sullivan.


The River Forecast Center in Minneapolis knows how much water is moving through the river when it’s at a certain height, Kuhlman said. Then they know how much water will hit the ice jam and how far it will spread out around it.


Using computer models, the weather service also calculates how much water might end up in the river from ice or melting snow.


—Ann Marie Ames

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