1. Most Finnish streams were channelised during the 19th and 20th century to facilitate timber floating. By the late 1970s, extensive programmes were initiated to restore these degraded streams. The responses of fish populations to restoration have been little studied, however, and monitoring of other stream biota has been negligible. In this paper, we review results from a set of studies on the effects of stream restoration on habitat structure, brown trout populations, benthic macroinvertebrates and leaf retention.
2. In general, restoration greatly increased stream bed heterogeneity. The cover of mosses in channelised streams was close to that of unmodified reference sites, but after restoration moss cover declined to one-tenth of the pre-restoration value.
3. In one stream, densities of age-0 trout were slightly lower after restoration, but the difference to an unmodified reference stream was non-significant, indicating no effect of restoration. In another stream, trout density increased after restoration, indicating a weakly positive response. The overall weak response of trout to habitat manipulations probably relates to the fact that restoration did not increase the amount of pools, a key winter habitat for salmonids.
4. Benthic invertebrate community composition was more variable in streams restored 4–6 years before sampling than in unmodified reference streams or streams restored 8 years before sampling. Channelised streams supported a distinctive set of indicator species, most of which were filter-feeders or scrapers, while most of the indicators in streams restored 8 years before sampling were shredders.
5. Leaf retentiveness in reference streams was high, with 60–70% of experimentally released leaves being retained within 50 m. Channelised streams were poorly retentive (c. 10% of leaves retained), and the increase in retention following restoration was modest (+14% on average). Aquatic mosses were a key retentive feature in both channelised and natural streams, but their cover was drastically reduced through restoration.
6. Mitigation of the detrimental impacts of forestry (e.g. removal of mature riparian forests) is a major challenge to the management of boreal streams. This goal cannot be achieved by focusing efforts only on restoration of physical structures in stream channels, but also requires conservation and ecologically sound management of riparian forests.