What you're describing happened accidenally outside of Anchorage at Potter Marsh. It's now a great spot to go birding and see other wildlife. > Potter Marsh wa

What you're describing happened accidenally outside of Anchorage at Potter Marsh. It's now a great spot to go birding and see other wildlife. > Potter Marsh was formed when construction of the Alaska Railroad embankment limited tidal ingress to a bridge over Rabbit Creek. This embankment impounded the creek and consequently formed the marsh. http://dnr.alaska.gov/parks/plans/pottermasterplan.pdf http://dnr.alaska.gov/parks/plans/pottermasterplan.pdf