| Abstract/Description or Keywords |
The Skeena River, located in northwest British Columbia, is a large river with a mostly natural flow regime. The floodplain of the lower river covers approximately 16,500 hectares, and is geomorphically very dynamic. Large-scale erosion and deposition processes are constantly changing the location and ecology of floodplain landforms. The hydrology of the river is characterized by a spring/early summer peak discharge driven by snowmelt; most floods occur during this period. The spring/summer flooding regime contrasts with that of smaller coastal watersheds where fall/winter weather events have a greater effect on hydrology and geomorphology. Flooding is the main disturbance type on the floodplain. Flooding can influence ecosystems through inundation, sedimentation and erosion. The hydrological dynamics of the Skeena River floodplain are generally intact relative to those of other large river floodplains in North America. This is due to a lack of water control structures such as dams and dikes, though there have been some impacts from highway and railway developments. Disturbances by fire, wind, disease and insects play a minor role in structuring the ecosystems of the floodplain. The river has many large in-stream wood deposits, which are important to the riverís dynamics and island formation and stability. The logjams also provide important fish habitat. Ecosystems of the area are classified into three types based on flooding dynamics and the resulting vegetation: low, middle and high bench. These ecosystems form a dynamic gradient and successional sequence that is influenced by the flooding regime. The deposition of sediments over successive flooding events gradually builds up, raising the landforms and changing the flooding regime and growing conditions. The high bench Sitka spruce Ė Salmonberry ecosystem is Red-listed by the BC Conservation Data Centre and the middle bench Cottonwood Ė Red-osier dogwood ecosystem is Blue-listed. These ecosystems dominate the forested areas on the floodplain. Only a small proportion of the original high bench forest remains in an undisturbed condition. The amount of undisturbed middle bench is unknown due to difficulties in differentiating this type from harvested stands on aerial photographs. Many rare plant, wildlife, and fish species may occur on the floodplain, however, inventory information on most species is lacking. The floodplain plays a role in maintaining regional biodiversity by providing low elevation coniferous and deciduous riparian habitat where these habitats are not common, and in providing seasonal habitat for migratory species. Forest harvesting on the floodplain started in the late 19th century, and reached a peak in the late 1950ís and early 1960ís. Most of the resultant secondary stands are dominated by deciduous species, primarily black cottonwood with some red alder, and have a conifer component that is highly variable in density. Some of these secondary stands are now being harvested along with some primary deciduous stands. Attempts have been made to establish conifer plantations on the alluvial floodplain sites. Most of these attempts have failed because of intense competition from brush and deciduous trees, flooding, and animal damage. Present forest management practices include increasing the focus on deciduous species for crop trees and fibre production, and retaining existing conifer regeneration to provide future coarse woody debris and stand structure. Coarse woody debris is important on the floodplain in maintaining biodiversity and in supplying regeneration sites where there is intense competition from shrubs. Management of coarse woody debris will play an important role in the future regeneration dynamics of these floodplain ecosystems. |
