What would happen if a US domestic flight had to make an emergency landing in Canada?

It depends where the Airplane needs to land and WHY it is landing.?

An on board fire has a different on ground action then a passenger in medical distress.

For a minor thing the plane is kept isolated and everyone stays on board.

If the plane needs to evacuated they are taken to a secure area until the situation is dealt with. They may need to wait until Immigration/Customs Officials arrive.

Officials can process the people without passports. There are many ways to identify people.

To continue on after the incident if they have passed Canada Customs/Immigration then they will need to pass USA Customs Immigration before they can enter the USA again.

This happens frequently.

http://www.nydailynews.com/news/national/london-bo...

They are not left at the side of the runway.

Many reasons a flight can make an unexpected landing

http://www.bbc.com/news/world-us-canada-34275103

http://www.godvine.com/read/Story-From-A-Plane-Div...

Most larger Canadian airports have a Transborder terminal that has flights that are only US bound, and since US Customs does Pre-Clearance u would arrive at one of those gates and not be allowed to leave the secure zone

It happens all the time. The passengers are generally kept in the airport or in a controlled group until the plane can go again or a replacement plane comes. Many Canadian airports have US customs and immigration officers at the terminals.

They keep the passengers in a particular area of the airport. Not having passports is only an issue if they want to send the passengers through immigration/customs and out of the airport; if they keep the passengers in a "transit" area, then there is no need for passports. So if it's a short delay - or even maybe a single night delay - then they would just keep everyone in one area of the airport.

It would depend on how long they had to stay in Canada. If the delay was a few hours then they would just stay in a controlled area of the airport until they took off again. If they had to stay over night, they would take them to a hotel in a bus and just keep an eye on them.

What Dash said. If they have to spend the night, the Canadians will figure out a way to verify their statuses and work out something with US immigrations officials for the continuation of the flight to the US.

Do you know about "The Day the World Came to Town"?

Here's the book.

http://www.amazon.com/The-Day-World-Came-Town/dp/0...

There is a FANTASTIC documentary about it.

On 9/11/2001, many flights were grounded on their way into the USA. Over 10,000 people were on planes that were re-directed to Gander, Newfoundland, Canada. People weren't stuck on the plane then. And they wouldn't be now, either.