Why isn’t Gastineau Channel navigable all the way through?

On a clear, sunny day, a boat makes down the Gastineau Channel way on Wednesday, Oct. 12, 2016, in Juneau, Alaska. (Photo by Rashah McChesney/Alaska’s Energy Desk)http://s3-us-west-2.amazonaws.com/ktoo/2017/07/170721DREDGING-CJ-podcast-mix.mp3For most mariners Gastineau Channel is a cul-de-sac. Vessels coming into Juneau almost always have to exit the way they came in. The Juneau-Douglas bridge isn’t the issue – it’s just too shallow for most boats except small craft runn