When a Republican candidate for U.S. president said over the weekend that he would be open to looking at building a wall along the Canadian border, he sparked international headlines.

Wisconsin Gov. Scott Walker, pressed twice by an interviewer, made the comment as an aside during an interchange about migration from the U.S's southern neighbour, Mexico.

That issue has moved front and centre among the field of Republicans running for president, as front-runner Donald Trump consistently floats anti-immigration ideas like building a Great Wall of Trump along the Mexican border and deporting 11 million migrants.

But a wall abutting Canada, along the longest international border in the world? For starters, it would be hugely expensive to cover the 8,891 km of frontier, 40 per cent of which is water — probably on the order of $30 billion, extrapolating from the $3.2 billion cost of building just 1,040 km of border wall with Mexico.

But more than that, it would be a logistical nightmare. An opera house would be forced to close, people's kitchens would be cut off from their living rooms, farmers would be stranded without access to roads, a half dozen airports would have to shut down— even a golf course would lose all its players.

The fact is, the Canada-U.S. border, which evolved and shifted over decades through a series of wars, treaties, disputes and resolutions, is full of quirks.

Here are some of the ones that would make a throbbing headache of any attempt to wall it off.

Airports

Piney Pinecreek Border Airport

The lone runway at Piney Pinecreek Border Airport spans the Canada-U.S. boundary between Manitoba and Minnesota. It's one of a handful of border-straddling airports. (Google)

A handful of airports on the Prairies and in B.C. straddle the Canada-U.S. border, including four where the runway itself either runs right on the border or bisects it. Some are really rural airfields with grass runways, but they have customs offices and official airport designations.

Now, imagine trying to land a Piper Cherokee while hurtling toward a border wall that cuts through the runway about halfway along. That's what pilots would face at one of them, Piney Pinecreek Border Airport in Piney, Man., and Pinecreek, Minn.

Farmers

The late New Brunswick farmer Nikolaj Pedersen had his share of headline-grabbing snarls with the Canada-U.S. border, and a wall along it would have only made it that much worse.

Nikolaj and Marion Pedersen lived for decades on a farm near Perth-Andover, N.B. Their fields, barns and house were all in Canada, but drive west to the end of their driveway, and you cross into the United States mere centimetres before turning onto the only road. That road runs a few hundred metres north back into Canada.

U.S. Customs showed them little mercy, however, and required anyone visiting their home — children, the local MP, the mail carrier — to take a 25-minute detour to the south to check in at the nearest American port of entry in Ft. Fairfield, Maine.

"There is a road that goes along the U.S. side of the border. There's a U.S. house down on the U.S. side and Canadians down on the Canadian side, and that's technically in Canada, so the people that live on those roads, they know that they have to check in with us as well," U.S. Customs officer David C. Wentworth said. Even borrowing an egg or cup of flour from a neighbour across the street could subject someone to arrest and a stiff fine for illegal entry into the U.S.

Province Point, Vermont

An aerial view shows the Canada-U.S. boundary running through Province Point, Vt., whose only land access is to Clarenceville, Que. A farmer owns the land and cows apparently graze there. (Google)

Another border-straddling farm property in Clarenceville, Que., hasn't had such conflicts over the years, but its cows would be cut off from some of their pasture if a boundary barrier went up. The farm stretches south across the border into a peninsula known as Province Point, Vt., which is an exclave —it's only connected by land to Canada.   

Golf course

A few hundred metres south of the Pedersen property in New Brunswick lies the Aroostook Valley Country Club, a golf course that did wonders for American duffers during Prohibition. They could play 18 holes and enjoy a beer (or two or three), then head back to the liquor-free U.S.

These days, the holes and clubhouse are in Canada, while the pro shop and parking lot are on the American side of the line. Both countries used to have a border station nearby for members to check in at, but the U.S. closed its outpost, despite protest, and now requires Canadians to check in at the Ft. Fairfield, Maine, port of entry.

Americans who come from the U.S. to golf don't have to report back to U.S. Customs on their return home, though, even though they've technically been hitting the links in Canada.

One reason the U.S. is so strict here: in 2007, an American man was charged with buying drugs from a Canadian somewhere on the front nine, which he hid with his golf balls.

Opera house

Haskell Opera House in Stanstead, Quebec

A black line along the ground (lower left) demarcates the Canadian and American sides of the Haskell Opera House, which sits on the border in Stanstead, Quebec, and Derby Line, Vt. (Haskell Free Library and Opera House)

A Canada-U.S. border wall might be felt most acutely in Stanstead, Que., and Derby Line, Vt. There, the Haskell Free Library and Opera House building straddles the border.

A black line runs across the floor of its reading room to show the boundary — book stacks largely on one side, the library's main entrance on the other — and also along the floor of the front rows of seats in the performance hall. Build a wall along it all, and the books would be separated from their readers, while the arias would have little audience.

Gas station

Gaz Bar U.S. in Estcourt Station, Maine

The only public road leading to Gaz Bar U.S. in Estcourt Station, Maine, is a street that runs along the border in Pohénégamook, Quebec. U.S. officials require Canadians to check in with American customs before filling up. (Mrgriscom/Wikimedia)

The handful of residents in Estcourt, Maine — and even the local U.S. Customs bureau — have Quebec area codes. They get their water and electricity from Canada. Their neighbours on Rue de la Frontière in Pohénégamook, Quebec, are Canadian, but their homes straddle the border.

"My house is in Canada, but my patio and my backyard are totally in the U.S. I even have a garage in the backyard," said Raymond Trudel, a semi-retired butcher who was born a few doors down from where he lives.

"The beauty of it is we cross the border every morning and night, but they don't make us go through customs. It's tolerated that they let you do what you want on your land."

But put a wall through it all, and that happy binational existence will come to a crashing halt. "I wouldn't even have access to my yard or my garage," Trudel said.

The tight clustering of American and Canadian people and property has almost never caused problems here, with one big exception: in 2002, Canadian Michel Jalbert turned off of Rue de la Frontière into the Gaz Bar U.S., a gas station about 50 metres into the U.S. whose only public access road is from Canada.

Jalbert hadn't checked in at the U.S. border point down the road, as signs outside the gas station warn drivers to do. After filling up, he was stopped and apprehended by two U.S. border officers and spent 35 days in jail before charges of illegal entry into the U.S. and possession of a firearm were resolved through a guilty plea (he had a hunting shotgun in his car).