MONTPELIER, Vt. (AP) – Minor ice jams caused water backup Thursday on the Winooski River, driving the river to its highest levels yet and prompting officials to warn of a “substantial” risk it could flood Vermont’s capital.

Hours after saying they believed the threat of flooding was diminishing, city and state officials revised their forecast, and the National Weather Service extended a flood watch through this morning.

“There was a general feeling among everybody involved – the Corps of Engineers, Vermont Emergency Management – that we’re not quite out of the woods yet,” said Greg Hanson, hydrologist for the National Weather Service.

“We were hoping the river would crest later this afternoon. But there’s some snowmelt and water that has to get past Montpelier. Until that actually happens and we see flows diminish and river levels drop, we need to keep our guard up.”

River ice has cracked in places and minor ice jams have caused some water to back up, with the river level rising three feet throughout the day to 10 feet – 5 feet below flood stage – as of 4:45 p.m.

Temperatures fell into the 30s, and the forecast called for below-freezing temperatures through Monday, which officials said would reduce snowmelt.

For weeks, the capital has been on edge, fearing a repeat of a March 11, 1992 flood caused by an ice jam on the river. Vermont National Guard members have filled and distributed thousands of sand bags, and city officials have warned residents to prepare for the worst.

The threat was exacerbated Wednesday night as temperatures climbed into the mid-50s and a steady rain fell, but it wasn’t enough to break the river ice into chunks that could cause a jam, officials said. Less than a half inch of rain was recorded.

But an ice jam on the Dog River that emptied into the Winooski on Thursday changed the forecast, prompting water backups.

American Red Cross workers walked streets downtown, handing out “Are you ready for a Flood or a Flash Flood” fliers with instructions on how to prepare, evacuate and cope with flooding. “We just want people to be prepared,” said Linda Runnion, director of development for the Central Vermont-New Hampshire chapter, who approached pedestrians along State Street, pressing the fliers into their hands.

“Because of 1992, they remember,” said Runnion. “Plus, they’ve seen the TV re-runs of boats floating down the street” in the 1992 flood.

Pam Brouard, 44, of Richmond, who was in Montpelier to take her son to a music lesson, took one. She said she was leery about going to Montpelier and was keeping an eye on the river.

“We’re hoping to get out of Montpelier soon,” she said.

Montpelier wasn’t the only place with trouble.

Thawed by days of warm weather, the Mad River was a sea of ice chunks and broken tree limbs, overflowing its banks in Moretown and forcing volunteer firefighters to evacuate one house and rescue a woman who tried to drive her Subaru through a flooded road.

Part of Route 100B was under about 8 inches of water, and it was closed for a period.

“The water gets pretty deep and it messes with the structural integrity of the road,” said Washington County Sheriff’s Deputy Toby Bernier, who manned a roadblock, keeping vehicles from driving into the flooded roadway.

In Chelsea, a small private school was evacuated as a precaution after the White River flooded because of an ice jam, washing out Route 110.

Creek Road in Middlebury was also flooded, from nearby Otter Creek, officials said.

“Anything called “Creek Road’ or “River Street’ is generally a threat when water rises,” said Hanson.