Why was for the HVDC Quebec - New England Transmission as you can read on http://en.wikipedia.org/wiki/Quebec_-_New_England_Transmission , a cable tunnel was built and the just few year old overhead line crossing dismantled? Building a cable tunnel under a river is the most expensive way to realize a powerline crossing of a river and only suitable in densly populated urban areas. Building an overhead crossing of a wide river is cheaper, but also still expensive. In my opinion it is not economic to build first an overhead powerline crossing with 2 100m+ towers and replace it just after a few years by a cable running through a tunnel?

Wrong forum... I think you meant for the Canada forum.