British Columbia is a province of 95 million hectares of which 61 million hectares are forest land. BC's forests extend from just south of 49 degrees north to 60 degrees north; from temperate rain forest in humid low elevation coastal areas to arid subalpine forests in continental areas; from semi-desert and savannah forest conditions to boreal.
The Ministry of Forests considers 52 million hectares to be productive forest land or land that is biologically capable of producing commercial timber. When economic viability and environmental sensitivity are considered, about one half of this area is available for timber harvesting. The quality of forest land is highly variable with the most productive forest lands generally found on valley bottoms along the coast and in the interior wet belt.
Because of the variety of ecological conditions, forestry practices vary greatly across the province. In general, forest harvesting increases average runoff and total water yield. Clearcutting increases storm runoff volumes and advances the timing of floods. Small and moderately early autumn storms are most affected. Logging roads increase storm runoff and advance timing of floods. The road layout influences the response. ncreased storm and peakflows result in an increase of erosion and channel scour.
The major sources of sediments are landslides and bank erosion. Maintenance of sediment transfer is important for habitat. Forest landuse increases the sediment production by 2 to 70 times. Road surfaces and sides are most serious; streambank disturbance is serious and debris fans have major significance.
Changes in steep low order streams are readily identified. Detection in larger systems is more difficult. Road building and clear-cutting can alter the timing and magnitude of storm runoff events. These can cause changes in fluvial sediment transport. Land use changes may have more direct effects by making more sediment available for transfer.
Not only is streamflow affected but changes in sediment sources and mobilisation of sediment are superimposed on changes in streamflow. Five landform units are important: hillslopes, gullies, stream channels, riparian zones and valley floors. Primary sediment mobilisation occurs on hillslopes and in gullies; remobilisation occurs from storage zones and along stream channels. Estimates of the rates of mobilisation of sediment are controversial and the precise extent of the acceleration produced by forestry activities is unknown. Coarse sediment, fine sediment and large woody debris should be considered separately. Terrain stability has become an overwhelming concern in forest practice guidelines and recent legislation.