Dartz Wrote:But it'd also take just as long to build a thousand-kilometre long canal deep enough. It'd need to be a couple of kilometres deep at points.This is why locks were invented. Your canal may not flow in the direction you want, but you can still float traffic a fair distance between locks without having to dig kilometers deep.
Also, using locks presents us the opportunity to have the Martian equivalent to the the Boom Towns of the American West. These towns started popping up like daisies to support the westward expansion of railway lines - the most famous one of all being the Intercontinental Railroad which joined the growing eastern and western railway networks of the USA. I can easily imagine a similar undertaking on Mars and the potential for 'Wild West' fen.