Floodplains, flooding, and salmon rearing habitats in British Columbia: A review

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The purpose of this review paper was to examine the relationships between floodplains, flooding, and juvenile salmon habitats. A wide range of topics were explored, these include; defining and characterizing fish habitats and floodplains; explaining flooding and hydrological processes; outlining fish behaviours, diets, and ecology. The potential impacts of forestry, agriculture, and urban development on floodplain processes, fish stranding, and fish habitats were explored. This includes a short discussion on chemicals and pollutants that enter fish habitats as a result of human activities on floodplains. In developing an understanding of our current knowledge of floodplain habitats and in identifying significant knowledge gaps, over 500 references were cited. This review was focused on British Columbia’s floodplains but does contain references to research done elsewhere.

Social values exceed environment concerns on a developed floodplain and few viable options remain. A major flood can cause loss of life and tremendous property damage. Thus, humans tend to view floods with fear. From an ecological prospective flooding is a natural event and an integral part of salmonid life cycles. When a river is separated from it’s floodplain and held to a permanent course by dykes, considerable loss of salmonid habitat will eventually result. We can not maintain natural flooding patterns or regain many of the historic floodplain fish habitats, once floodplains have been urbanized. We can only protect the remaining critical habitats from development.

On an undeveloped floodplain, consideration can be given to limiting activities to those that are consistent with maintaining natural systems. It is important that we maintain the natural hydrograph, permit the flooding of floodplains, support the natural avulsion of a river channel, and protect wetlands. From an ecological prospective, floods are natural and are important in maintaining the health of the river, riparian zone, and floodplain. Fish and invertebrates are adapted to seasonal flooding. Periods of high water may serve as a queue for migration or an opportunity to move into and exploit different habitats. Floods create new channels and a succession of new habitats while eliminating others. Floods clean the substrate and alter the species composition of the riparian communities.

The wetlands associated with floodplains support the rearing of juvenile salmon. Floodplains provide habitat for juvenile salmonids in the form of seasonal wetlands, temporary tributaries, off-channel ponds, sloughs, flood-channels and seasonal estuarine drainages. Natural floodplains reduce the heights of floods, storing floodwaters in wetlands, and distributing the floodwaters over a wide area. They also filter storm waters, trapping sediments, nutrients, and removing pollutants. Floodplains are a major source and processor of litter. When compared to lentic habitats, these seasonal habitats support a different mix of invertebrates, usually have more modified water temperatures, and may have different water quality concerns. Many of these habitats support higher densities of juvenile salmon and have higher grow rates than main channel habitats.

Coastal and interior floodplain habitats are used by a number of regionally important salmon species. These fish appear to have adapted behaviours that enable them to successfully exploit seasonally flooded lands. Human activities such as forestry, agriculture, and urban development can affect salmonid floodplain habitats. Based on this review, some major gaps in our knowledge and concepts we should consider when examining floodplains are listed below.