The Susquehanna is constricted by many dams throughout its watershed, but there are four major dams on the lower main stem of the river: the Conowingo (completed 1928), Holtwood (1910), Safe Harbor (1931), and the York Haven Dam (1904) are the main contributors to the ecological strangulation of this system. The dams' failure to provide acceptable fish passage has almost eradicated the once historic migratory fish runs that used to extend all the way to the headwaters of the Susquehanna in Cooperstown, NY.