We all hate lampreys
When the St Lawrence Seaway opened in 1959, the engineers showed an abysmal understanding of the natural environment. Ships travelling around the world flushed ballast tanks containing invasive species, introducing gobies, zebra mussels and sea lampreys to the Great Lakes. The lake trout population plummeted, and given the fears arising after Hurricane Hazel, it was decided that a series of weirs would be built in many rivers to mitigate flooding, and also reduce lamprey reproduction. Like salmon and large trout, lampreys swim upstream to breed. The larger fish, in theory, can easily jump over the weirs, but the lampreys, not so much.
Nothing is ever that simple, but eventually weirs were notched, and fish ladders were developed that do allow the salmon to pass. The entrance to such a ladder is visible if you look across the river at the base of the weir. Ladders and lamprey traps are of necessity covered, both to prevent people falling in, but also to protect the fish from predation.
TRCA completed a study of the Humber weirs recently, and decided that they are not all needed, but the cost/benefit of removal isn’t favorable, so the weirs will slowly degrade and be assessed for safety but removal is not considered a priority.
http://www.glfc.org/sea-lamprey-lifecycle.php