IW11 Rivers without barriers

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Development

 This indicator looks at the opportunities for fishes to migrate in rivers. Free-flowing river is defined as the portion of the river from the mouth of the river that can be reached without bypassing any migration obstacles. The remainder of the river is classified according to the type of the obstacles present: blocked river, interfered river or a river behind a fish passage. Rivers over 30 kilometers in length are taken into account.

The damming of rivers was most intense in Finland in the first half of the 20th century, when the proportion of free-flowing rivers decreased rapidly. After the 1980s, significant dams were no longer built. Free-flowing river sections currently account for approximately 26% of the total length of rivers in Finland.

In the late 1980s, the first fish passages were built to reduce the adverse effects of dams. Today, about 15% of dams have fish passages, and about 7% of the length of rivers can be reached only through them.

impact on biodiversity

The damming of rivers prevents both migrant fishes from reaching the breeding areas above the dams and the migration of fishes back into the sea. Damming can also weaken the status of watercourses in other ways, for example by eradicating spawning habitats. Almost all migrant fish species in Finland are either endangered or vulnerable.

The purpose of the fish passages is to allow the fish to bypass the dam. A well-implemented fish passage can also provide favorable habitats and breeding areas. On the other hand, the benefits of poorly implemented fish passages may be negligible. And despite the passages, some of the fish that migrate back to the sea may die in power turbines. In addition to fish passages, the adverse effects of dams on fish have been alleviated by imposing fish stocking obligations on dam owners.