Rivers are water corridors that flow through landscapes, connecting very distant points from the headwaters to the sea.
Aquatic organisms move and disperse along these corridors, populating river ecosystems. Connectivity is a fundamental constituent of river system, at the point that some fish species exploit it during their life cycle. For instance, the Adriatic sturgeon (Acipenser naccarii) spends most of his time in the lowland sections of rivers but migrate upstream during reproduction to find suitable sites where to lay eggs.
The connectivity of river systems is altered by barriers used to exploit rivers. Weirs are built to control the flow for irrigation purposes on small lowland streams. Check-dams to stabilize the river bottom in mountainous streams. In the main channels, dams are built for hydroelectric production, water storage for irrigation, and for flood control. All these barriers prevent (partially or totally) the movement of organisms between different sections of the rivers.
The impacts of barriers construction on freshwater migratory fish are severe. Some fish populations might be prevented from reaching specific habitats they need to complete their life cycle (e.g. reproduction), and will eventually disappear. Furthermore, the reduced exchange of individuals between isolated populations could lead to genetic drift and to less resilient populations.