The main goal is to revive migratory fish stocks
Most of the rivers in Finland are modified rivers, in which stretches of various lengths have been straightened, cleared and altered for purposes of timber floating and hydropower generation through the adding of necessary structures. In modified water bodies, the main goal of restoration by 2015 is to promote the natural breeding of migratory fish species in 30 river basins, in accordance with the Water Management Implementation Programme. The key objective of river restoration in Finland is to revive migratory fish stocks by improving fish habitats.
Sea troutand migrating whitefish are classified as Critically Endangered and Finland’s indigenous salmon stocks as Vulnerable. Fish habitat restoration usually involves various combinations of methods used in the restoration of flowing waters. Fish stocking is often carried out in restored rivers to restore the natural life cycle of the fish.
River habitats can be restored through a variety of methods. Examples include increasing the structural diversity of the channel, protecting the banks from erosion, and facilitating fish movement by removing obstacles to passage. Some 400 river restoration projects have been completed in Finland so far, many of which were connected to the end of timber floating. One of the goals in nearly all of these projects was improving the living conditions of fish.
Structural diversity increases species richness in rivers
Each body of flowing water has its own history. Over time, natural river channels have adapted themselves to prevailing conditions. The structure of a river – i.e., the features and shapes of the river and its surroundings, which can be natural or man-made, together with the flow rate and water quality, affect the action of the river and the organisms that thrive there. All of these elements are vital. Research has shown that enhancing water quality alone, for instance, is not sufficient to improve species richness in flowing waters. This is why attention should be paid to the network of interaction at the level of the entire river basin.
River basin planning should take into account the features of the natural river or stream. These include fluctuations in water depth and river width, along with pools and riffles. Wood and stones at the bottom of the channel help to create areas of varying depth and various flow conditions. The free movement of organisms, flooding of the channel and diverse riparian vegetation are characteristic of natural flowing waters. In addition, the course of a natural river meanders and changes somewhat, creating new habitats at old bends. The objective is therefore a diverse environment providing favourable conditions for as many species as possible.
The target state of a river basin can be determined by surveying nearby natural channels or by making use of old information, such as old maps. Surveying natural features assists in the planning of river basin restoration work. Although the current land use on banks and the conditions of land ownership often limit extensive restoration work, they do not rule out smaller-scale measures, such as planning of diverse riparian vegetation or a cross-section that imitates a natural channel.
Environmentally sound hydraulic engineering reconciles multiple pressures
River restoration and the pressures of use facing rivers can be reconciled through environmental hydraulic engineering. This means restoring a river channel in such a way as also to enable meeting the demands imposed on the channel or river basin by hydropower generation or land drainage, for example. Since many traditional hydraulic engineering projects can today be implemented by paying more attention to the natural state and landscape of the river ecosystem, environmental hydraulic engineering should be integrated into the planning and construction processes for all waterway projects.
The key principles of environmental hydraulic engineering include
- preserving and restoring natural discharge ratios,
- preserving and restoring natural and self-sustaining structures in river channels and on river banks,
- reducing loading from the river basin, and
- restoring and ensuring the diversity of biotic communities.
Restoration work should take place during the low-rainfall season, when the water level is clearly below average. The fish population of the river should always be taken into account in the restoration planning. The spawning and egg-hatching times of salmon and crayfish must definitely be avoided. In addition, restoration should take place outside the crayfish and river lamprey trapping seasons. If there are fish farms in the vicinity of the area to be restored, it is recommended that restoration be carried out in the winter.