It was 2017 when Canadians first began to hear of an obscure border crossing near Hemmingford, Que., that was witnessing an unusually high number of illegal crossings on foot.

Large numbers of Haitians, many of them families dragging heavy suitcases, were flying from homes in Florida and New York City to upstate Plattsburgh, N.Y., and then taking a taxi to the tiny farming community of Mooers, N.Y.

Roxham Road would soon become a major news story and a political issue.

In March last year, Canada and the U.S. agreed to the first major change to the Safe Third Country Agreement (STCA) in 20 years, expanding its terms to cover the entire land border rather than just formal crossings.

That STCA expansion effectively ended Roxham Road's usefulness to migrants as an end-run around that system.

But one lesson from the Roxham Road experience remains highly relevant and concerning in light of former U.S. president Donald Trump's promise to deport millions of immigrants if he wins a second term: any changes to the status of undocumented migrants in the U.S. can be swiftly felt at the Canadian border.

A minor change causes a rush at Roxham Road

The Roxham Road influx was triggered by the ending of an obscure provision of the U.S. immigration system that had extended Temporary Protected Status (TPS) — immunity from deportation — to Haitians in the wake of the 2010 Port-au-Prince earthquake.

TPS was also cancelled for Central American immigrants who had been protected because it was deemed unreasonable to return them to an area devastated by Hurricane Mitch.

They too began to show up at the New York-Quebec border.

Although the TPS cancellation affected only about two to three per cent of undocumented migrants in the U.S., it was nonetheless enough to cause an influx into Canada that required governments to scramble to accommodate it, with Montreal even opening its Olympic stadium as a temporary refuge.

Trump has pledged to go after all undocumented immigrants in the United States.

The U.S. government estimates the total to be about 11 million people, while Trump himself put it at 18 million during his debate with President Joe Biden, and some on his team have cited the number 30 million.

Trump's first administration, by contrast, deported fewer than a million people.

Word spread by social media, community radio

At the height of the TPS cancellation rush, a dead-end turnaround at the end of Roxham Road on the New York side of the border was littered with bus ticket stubs and airline boarding passes from cities all over the U.S.

CBC News met people there coming from as far as San Diego.

Information about Roxham Road, some of it false, was being shared on social media and on Creole-language radio stations in Miami and Fort Lauderdale, Fla.

The Canadian government also turned to social media to counter those messages, and Creole- and Spanish-speaking government members travelled to Miami to discourage crossings.

"Cellphones and particularly smartphones and social media really are able to drive migration in ways that I think that many countries didn't anticipate, which is why you see such movement across the Western Hemisphere," said Laura Collins, a migration expert at the George W. Bush Center in Dallas.

Collins told CBC News in an interview a crackdown in one jurisdiction can often have the effect of pushing migrants into another.

"Obviously there would be people that would try. We've seen in Europe, when the U.K. implemented their harsh crackdown, people tried to cross the border from Northern Ireland to (the Republic of) Ireland," she told CBC News.

"I would say that it's likely people would try to come to Canada. I don't know what the scale would look like."

'The biggest deportation force this country has ever seen'

Tom Homan, an acting chief of Immigration and Customs Enforcement (ICE) in Trump's first term, has been touted as a possible Secretary of Homeland Security in a Trump comeback administration.

"They ain't seen s--- yet," he said of the deportation effort in Trump's first mandate. 

"Trump comes back in January. I'll be on his heels coming back, and I will run the biggest deportation force this country has ever seen."

Unlike many of Trump's policies, the mass deportation proposal polls well even outside his base.

Fifty-one per cent of Americans support the idea, including 42 per cent of Democrats, according to a survey of 6,251 adults conducted online in April by The Harris Poll, with a margin of error of plus or minus 1.5 percentage points using a 95 per cent confidence level.

(The same poll found many Americans are confused about basic facts of illegal immigration. Nearly two-thirds believe immigrants receive more in welfare and benefits than they pay in taxes. In fact, most U.S. undocumented immigrants pay taxes but aren't eligible for the benefits themselves.)

The deportation plan would actually bring the U.S. in line with practices in Canada and Mexico — both countries that routinely deport all foreigners found to be in the country illegally.

But neither Canada nor Mexico have anything remotely comparable to the vast undocumented population of the United States.

A promise unlikely to be fulfilled

If mass deportation is fairly popular with the American public, many experts don't believe the promise is achievable.

"The first thing I always tell people when they ask about Trump campaign promises to carry out mass deportations," Collins said, "is that it would be incredibly expensive, incredibly disruptive to the U.S. economy, it would take decades, and it would be almost impossible to implement."

"We don't currently have enough judges in the immigration system. We don't currently have enough ICE agents. We don't have enough detention facilities to hold any of the people that they would arrest. And before they are removed, they'd also need to work with the countries of origin for these folks to make sure that we have the relationships in order to remove them to their home countries," Collins explained.

Collins said many fans of the proposal haven't thought through its real implications, including the fact that over a million U.S. citizens are married to undocumented immigrants, and that many U.S. children have one or both parents who would be subject to deportation.

Rumours and fear

But while the reality of Trump's mass deportation plans may fall short of the sales pitch, as did the southern border wall he promised in 2016, that doesn't mean it won't cause panic and a potential rush, Collins said.

"It's one thing for me as a person who studies policy to look at it and say this is nearly impossible to do and people should calm down a little bit. But it's another thing if your actual family would be impacted by this policy. And I think it's really hard to estimate just how fearful you must be if you are an American citizen child and your parents are undocumented, or you're a Dreamer and you know your status is always tenuous because right now there's not a permanent legislative solution for you," Collins said.

"Those things are real impacts on real people, and it will determine how they behave in the next couple of years. And that fear is hard to quantify. And it's always worth remembering that when we think that policies are nearly impossible, that doesn't mean that they aren't very real and scary for real people."

There are several questions about how Ottawa and Washington might react should a mass flight of migrants from ICE dragnets cause a rush into Canada.

One unknown would be how the Trump administration would react when Canada invoked the SCTA to return all border-crossers to the United States, as is its right under the treaty.

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Would the U.S. accept them, particularly if they came from countries such as Venezuela where it has historically been difficult to return migrants because of a lack of co-operation from national governments?

Would a Canadian government face blowback at home for turning migrants over to a Trump administration's deportation force?

If the influx were sufficiently large, a significant number of migrants might be expected to qualify for one of four exemptions to the STCA. How would Canada absorb those people?

The U.S. has seen rapid growth in the numbers of unaccompanied minors crossing the U.S.-Mexico border, to take advantage of exemptions in U.S. policy that allow them to stay.

The STCA offers a similar exemption to children travelling without adults. Might Canada potentially see the same phenomenon?