Calgary will not see the next CTrain line run under the Bow River.

City council had approved a four-kilometre Green Line tunnel to run from 16 Avenue North, beneath the river and under downtown.

Due to technical challenges, that plan must change.

"We believe that the risks of going underneath the river with the tunnel that we were looking at and the station construction we were looking at are too great," transportation general manager Michael Thompson told CBC News.

Instead, the city will resurrect a discarded idea to build a bridge across the Bow. That would connect the Green Line from Crescent Heights into a tunnel under downtown.

Downtown tunnel still a go

The details around the connection to a downtown tunnel have yet to be determined, Mayor Naheed Nenshi said.

"It's pretty clear that it has to go underground downtown — unless there is some elevated technology that I don't know about — but it can't run at grade," Nenshi said.

A bridge matched with a downtown tunnel may be the lower-risk, lower-cost option, he said, but that analysis is still underway. City planners are dedicated to getting the design done properly before breaking ground, he said.

"This is a very long-term project, regardless," Nenshi said. "Let's make sure we get it right from the beginning."

Because the technical issues need more work, construction on this section of the Green Line is not slated to begin until 2022.

The opening of the $4.6-billion project may be delayed to 2027 due to unrelated contract changes.