Complex sentence: The dam creates an obstacle for the river herring that run in the Spring when the high concentrations of Blueback herring and Alewife below the dam make it a popular place for seagulls, Great Blue Heron, Night Heron and Cormorants to fish.
Split into simpler sentences: The dam creates an obstacle for the river herring that run in the Spring. The high concentrations of Blueback herring and Alewife below the dam make it a popular place for seagulls, Great Blue Heron, Night Heron and Cormorants to fish.