Yep, that is the channel blocked off at Templar's Island!

And if you get onto LINZ website, and grab their LIDAR data and visualise it, you can see all the old channels it has carved across the Plains, including ones right through CHC airport into the headwaters of the Avon (this is the route the Waimakariri took when it flooded Christchurch in the late 1800s).

As for Banks Island, well it was an island back in the day, not sure how many thousand years ago that was though.

PS, if you're in ChCh and tech, feel like we might know each other... love how small the net can make the rather large world.