Police have confirmed that a three-year-old boy is missing in the Orangeville, Ont., area after a van left a roadway and wound up in the Grand River as southern Ontario communities deal with flooding.

The child slipped from his mother's arms as she was getting out of the submerged van, according to the Dufferin detachment of the Ontario Provincial Police.

"Unfortunately, the three-year-old son was swept away in the water downstream," said Const. Paul Nancekivell.

Heavy rains and a deep thaw are behind the flooding and high water along rivers and streams across southern Ontario, with the Grand River watershed seeing the worst of the effects so far.

A state of emergency was declared Wednesday in Brantford, after parts of the city were evacuated due to flooding along the Grand River.

"The levels and height of the water are something that have not been seen in this community in a very, very long time," said Brantford Mayor Chris Friel.

Boy swept away in river

Dufferin OPP said that just before 1 a.m. ET, a female driver passed a roadblock put in place because of flooding. Her van was swept into the river near Orangeville. As the woman was exiting the vehicle, the child slipped out of her arms. The mother was taken to hospital.

The intensity of the fast-moving river swept the vehicle downstream about two kilometres toward a bridge at Line 109 and Tenth Line to its resting point.

Nancekivell told CBC News that "it's an ongoing police investigation." He said the OPP are searching for the child with a helicopter, a dive team, a drone and other search-and-rescue responders.

Downstream alerts

The flooding is expected to get worse downstream from Brantford later today, in places like Cayuga and the Six Nations reserve.

Reports of flooding stretched across southern Ontario, including London, Waterloo, Cambridge, Dunnville, St. Marys, Chatham-Kent and Orangeville.

Officials said ice is blocking water and forcing the heavier flows to find ways around the jams.

In Six Nations, the elected council called a state of emergency late Wednesday afternoon.

Residents along 744 West to Bateman Line were most vulnerable, the council said.

"Please note that the current flood conditions are not typical of Six Nations of the Grand River Territory," the council said. The highest water levels are expected to occur overnight Wednesday.

Six Nations fire officials also closed roads: Fourth Line from Seneca Road to Bateman Line, and Mohawk Road to Third Line. The council is monitoring water levels around the Chiefswood Bridge.

Brantford evacuation orders

In Brantford, an emergency announcement Wednesday morning noted there is ongoing flooding in the neighbourhoods of Holmedale, Old West Brant and Eagle Place. Anyone living in those areas was urged to evacuate their homes.

"It's important to keep in mind the levels are very much tied to the ice jam," said Grand River Conservation Authority engineering director Dwight Boyd.

Parts of Brantford, Ont., are under an evacuation order because of rising water levels. (Annie Poulin/Radio-Canada)

"How it erodes or releases will determine how quickly levels will recede."

He said levels were stable as of 11 a.m. but that they would remain high through the day and into tomorrow.

Police went door-to-door in the affected neighbourhoods, asking people to leave immediately. Up to 4,900 residents live in the evacuation area.

"Buses are available to help evacuate @ Eagle Ave & Foster St., Baldwin & Erie Ave, Erie & Aberdeen Ave. Those requiring shelter can go to Woodman Comm. Centre," the tweet advised.

The City of Brantford has declared a state of Emergency due to the ongoing flooding crisis impacting the neighbourhoods of Holmedale, Old West Brant and Eagle Place. Public in those areas are STRONGLY URGED to evacuate immediately. Evacuation Details: <a href="https://t.co/HfpxaZ3q86">https://t.co/HfpxaZ3q86</a> —@CityofBrantford

Jeff Whitbread was one of the residents who left his home. He told CBC News that the process seemed to be handled well.

"The police and services have been around our neighbourhood and talked to all the residents and its very calm. It seems to be pretty well organized.

"We're just going to pick up a few things and head on out to a friend's house."

Residents were being relocated due to flooding along the Grand River after an ice jam upstream of Parkhill Dam sent a surge of water downstream. (Aaron Vincent Elkain/Canadian Press)

The Grand River Conservation Authority said Wednesday morning "an ice jam upstream of Parkhill Dam released at approximately 2 a.m. Wednesday morning. When this ice jam released, it sent a surge of water downstream."

Province 'ready to provide support'

Ontario Premier Kathleen Wynne toured the area on Wednesday and said the province is "ready to provide support."

"I think it works the best when the municipality lets us know what's needed so we can respond appropriately," she told reporters at the Brantford fire hall.

Wynne also said the Ministry of Transportation will inspect city bridges on Thursday and assess if they will remain closed.

Ontario's Premier Kathleen Wynne toured the area on Wednesday, pledging support from the province. (Paul Borkwood/CBC)

Deputy fire chief Andrew Lillico said the city is "not out of the woods" yet, as a second swell is expected mid-morning Thursday when more water comes down from the Nith River, which feeds the Grand.

At least 25 families will be spending the night at a local community centre, said Maria Visocchi, Brantford's director of communications.

"We are overwhelmed by the generosity of the local community in terms of food and water and toys and supplies for the children that are there," Visocchi said.

'You certainly don't want to see anyone get hurt'

The agency warned the public in a post on its website to exercise extreme caution around all water bodies.

"Banks adjacent to rivers and creeks are very slippery at this time and, when combined with current weather conditions, pose a serious hazard. Parents are encouraged to keep their children and pets away from all watercourses and off frozen water bodies, which will be extremely unsafe as a result of the warming trend."

Those who live in the areas identified in the floodplain map attached are strongly urged to evacuate. <a href="https://twitter.com/BrantfordPolice?ref_src=twsrc%5Etfw">@BrantfordPolice</a> are going door to evacuate the following streets Foster, Cayuga, Aberdeen, Strathcona, Pontiac, Tecumseh, Ontario, Port, Eagle Ave, Robertson, Denver and Baldwin <a href="https://t.co/j5AkP734QT">pic.twitter.com/j5AkP734QT</a> —@CityofBrantford

Friel said a dike system put in place in the 1980s prevented a much worse situation.

"This would already be a disaster in our community if we did not have the dikes that are in place."

Robert Putt, a resident in the evacuation area, said he woke up around 2 a.m. to the sound of trees breaking and snapping. When he got up this morning the water level had increased by about a metre.

"You're hoping not to lose property or value or have things damaged, you certainly don't want to see anyone get hurt," he said.

Putt said he's lived in his house for almost 30 years and has never seen flooding like this.

"It's a large amount of water and large amount of ice."

The following schools are closed because of the flooding:

Princess Elizabeth.

Agnes Hodge.

Jean Vanier.

Bellview.

Ecole Dufferin.

Ste Marguerite.

Ste Marie.

Voilà le flot de l’eau et de la glace sur la rivière Grand à Brantford. <a href="https://twitter.com/matintoronto?ref_src=twsrc%5Etfw">@matintoronto</a> <a href="https://twitter.com/hashtag/floodwarning?src=hash&ref_src=twsrc%5Etfw">#floodwarning</a> <a href="https://t.co/aXcUUgOanU">pic.twitter.com/aXcUUgOanU</a> —@Annie_Poulin

Updated flood warnings

The Grand River Conservation Authority updated its flood warning Wednesday for townships and cities including:

City of Cambridge

Township of Wilmot – New Hamburg

Township of North Dumfries - Ayr

Brant County

City of Brantford

Six Nations/Haldimand County – Caledonia

Haldimand County – Cayuga

Town of Dunnville/Port Maitland

Township of Puslinch/Wellington County

Town of Grand Valley / Waldemar

Township of Mapleton/Drayton

Township of Woolwich / St. Jacobs / West Montrose

In London, the Upper Thames River Conservation Authority has issued a flood warning and the city closed several roads due to rising water levels.

The Upper Thames River Conservation Authority said the watershed received 10 mm of rain since 4 a.m. Wednesday and another 10 mm was forecast before the end of the day.