Erosion
Erosion and transport of sediment, within the Greater Victoria Water Supply Area, is a key risk to water quality. Sediment, dissolved minerals, and organic material eroded from the land surface, or from stream channels, enter streams, and may be carried to the reservoir or into a water intake, where they elevate the turbidity and colour in the drinking water. The sediments that enter streams may be carried either as suspended, or wash, load or as bed load.
Fine sediment (clay and silt) is moved as “wash load” maintained in suspension by turbulence. Once in a stream, these sediments are usually carried to a reservoir, though some of them may be deposited in lakes, areas of slack water, secondary channels, on floodplains or deltas. Once in the reservoirs, these fine sediments are carried towards the intakes, though settling results in deposition of a large portion of them on the bottom of Sooke Reservoir (Barraclough, 1995).
Larger sediments (sand, gravel, cobbles, and boulders) are also eroded from the land surface and they are transported as bed load. These materials are generally deposited in the lower reaches of streams, or on their fans or deltas, and do not directly contribute to turbidity in the reservoir. However, they may contribute indirectly, by causing stream bank erosion or channel avulsions that ultimately erode those sediment sizes that cause turbidity in the reservoir.
In the Water Supply Area, sediment is mobilized by different erosion stressors, as described in the sub-sections below. Natural or human-induced disturbances in the watersheds are the source of these stressors, and the type or degree of disturbance changes the relative risk posed by erosion. Fire or pest infestation, which kills trees or expose mineral soil on stability class III, IV and V terrain units that are underlain by thick tills, may greatly increase the incidence of landslides and the fine sediment yield from surface erosion. The most susceptible areas in the Water Supply Area can be identified from the Surface Erosion Potential and Slope Stability Maps (Walmsley, 1990), although these maps do not indicate the increase of fine sediment yield that might occur after disturbance.