A fence would be impractical, for example, in places where it would
impede wildlife migrations, such as the wildebeest that chase the rains across the Serengeti every year.
Dams damage rivers: Dams disrupt flows, degrade water quality, block the movement of a river's vital nutrients and sediment, destroy fish and
wildlife habitat,
impede migration of fish and other aquatic species, and eliminate recreational opportunities.