Luonnonmukainen elinympäristö ekologisena kompensaationa virtavesissä : Imatran kaupunkipuron suunnittelu, toteutus ja seuranta

Built nature-like habitat can promote the juvenile production of migratory fish in vicinity of a hydro-power plant or a distinct dam structure. At the same time, the habitat serves as the basis for the entire ecosystem. Typically, the construction of hydropower has resulted in a significant loss of habitat suita-ble for aquatic life, which can be replaced by nature-like solutions as part of ecological compensation. The measure can therefore improve the ecological value and status of the human-modified water body.
This report describes the design and construction phases of the Imatra City Brook and reports the results of monitoring after the construction phase. The design of the Imatra City Brook emphasized that the brook will become suitable habitat for trout. However, brook is not designed as a bypass, albeit the de-sign principles used can be applied directly to natural bypasses.
The Imatra City Brook was completed in 2014 in a park-like area in the vicinity of the Imatrankoski power plant. The brook is about one kilometer long and varies in width from 2 to 10 meters. In addition to different habitats and spawning and juvenile areas for trout, landscape design and suitability for recre-ational use were also taken into account in the design of the area. The height of the Imatrankoski dam, 24 m, and the available area were utilized to make the best possible use, and by meandering the brook, its length could be maximized. Gravel of varying sizes, larger stones and wood were used as the base material for the structures. The flow of the brook is about 250 l/s in summer and about 100 l/s during winter.
The goal of the stream being a spawning and rearing ground for trout seems to have been achieved, as trout and other species of fish and organisms have taken over the stream. In the Imatra City Brook, trout reproduction has been even better than expected and the density of juvenile fish is higher than in the compared natural streams. The total area of the brook is remarkably small compared to the huge amount of rapids and riverine habitats lost for hydropower production and construction of dams. However, the built natural-like habitat provides a suitable habitat for trout as an endangered species and any individ-ual born in the wild can be considered important for the population of the species. The species richness of benthic macroinvertebrates increases every year. At the same time, the number of macroinvertebrate individuals has multiplied during the four years of observation.
Reducing damage to nature, species and ecosystems is becoming the responsibility of existing hydro-power. Fisheries compensation alone (stockings) or the technical fishways alone are not enough to solve the problems that hydropower and damming have caused, especially for the quantity and quality of suit-able habitats. In the future, the results from the Imatra City Brook will encourage the upgrading of vari-ous built natural-like habitats or compensatory habitats as one viable option for improving the status of the water bodies.