Alaska's Freshwater Coastal Jet

High rates of precipitation--up to 340 inches of water per year--along the coast of southeastern Alaska and British Columbia may have important local consequences for fish and marine mammals inhabiting the coastal waters there.

As rains occur and snow melts, there is massive runoff of fresh water into the Gulf of Alaska. No one is certain how much fresh water runs off the mountainous coast since not enough of the stream flows in the area have been gauged. However, Tom Royer of the University of Alaska's Institute of Marine Sciences estimates that the runoff easily exceeds by three times the flow down the Yukon River. It is even larger than the flow down the Mississippi River, estimated to average 23,000 cubic meters per second (810,000 cubic feet per second).

A curious thing happens to the fresh water running into the sea. Instead of flowing out southwesterly into the Gulf of Alaska and disappearing into the saltwater there, the fresh water forms into a cohesive jet that hugs the coast and flows northerly and westerly along it. There are two reasons for this behavior.

Firstly, the fresh water is lighter than ocean water and, because of the difference in density, the fresh and salt waters tend not to mix easily. Though enough mixing does occur to make the fresh water too salty to drink, an identifiable stream of fresher water is maintained.

Secondly, this stream turns to the right as it moves, as do all moving flows of fluid in the northern Hemisphere. The Coriolis force causing this turning is due to the earth's rotation on its axis. Thus the freshwater stream, roughly 10 miles wide, tends to hang against the coastline as it moves along at a speed of approximately two knots. Boat captains coasting northward can use the stream to speed their passages.