The National Weather Service issued flood warnings, which expire at noon today, warning residents about storms capable of producing heavy rain, up to 2 inches an hour in some areas.
This already is the wettest August in 20 years, the Illinois State Water Survey reported Friday, so such a downpour on saturated ground, combined with the 2 to 3 inches that have fallen since Thursday afternoon and all the water rushing down from Wisconsin tributaries, can combine to produce a runoff that leads to dangerous flooding.
In Como, a village along the Rock west of Rock Falls, Paula Payne looks out on a 40-foot backyard nearly covered in water. Luckily, her house is on a hill and not in any apparent danger. Nearby, a number of campers pulled their RVs away from danger a few days earlier.
"I've lived in this house my whole life, and I've never seen flooding like this in August," Payne said.
In Byron, the Rock reached moderate flood stage at 1 p.m. Friday; it should crest around midnight Sunday at levels just below major flooding.
Ogle County's Office of Emergency Management recommended that people living in low-lying areas along the Rock and along Kilbuck Creek in the Lindenwood area evacuate.
As of Friday morning, rainfall in counties from Boone to LaSalle and eastward beat the 1987 record of 11.02 inches. Totals for June through August so far in the area averaged 20.02 inches, 8.91 inches above normal and beating the 1972 record of 19.26 inches, the Water Survey reported.
The U.S. Coast Guard allowed the Whiteside County Sheriff's Department to close the Rock River to boaters and all other users indefinitely on Friday, citing high water, fast currents and debris. Winnebago and Lee counties also have closed the Rock in their areas.
In addition to the danger of being on unpredictable waters during a flood, boat wakes can wash debris and water into already flooded backyards, futher damaging property.
Anyone caught on the river could be issued a citation by state officials, Illinois Department of Natural Resources Conservation Police Capt. Greg Hunter said.
"We're trying to educate the public ... It may look calm, but currents are moving quickly and there's no room for air," Hunter said.
River levels are expected to crest Sunday, the National Weather Service reported.
Elsewhere, Gov. Rod Blagojevich declared four northern counties - Cook, Lake, Kane and McHenry - state disaster areas.
In DeKalb County, the Kishwaukee River reached near-record levels, flooding nearby neighborhoods and forcing about 600 residents of DeKalb and Sycamore to evacuate.
Reach Malinda Osborne at (815) 284-2222 or (800) 798-4085, ext. 526.