Complex sentence: With the disappearance of Killer Dana, water circulation in the bay decreased and resultantly, as pollution from San Juan Creek continued to flow freely into the bay, and stay there for a longer period compared with if the harbor had never been built.
Split into simpler sentences: With the disappearance of Killer Dana, water circulation in the bay decreased. Resultantly, as pollution from San Juan Creek continued to flow freely into the bay, it stayed there for a longer period of time in comparison with that if the harbor had never been built.