The increase in bedload and woody debris delivery to coastal streams from landsliding is probably the most important factor in creating stream channel change following logging.
Increased landslide rates are attributed to many factors. Road building disrupts subsurface drainage, often creating points of water concentration. Road sidecast can overload and oversteepen already steep slopes. Tree removal increases the amount of rain that reaches the soil because of the loss of canopy interception, and also increases the rate at which snow melts during both rain-on-snow events and spring snowmelt. Soil strength is reduced by the decay of anchoring roots in the years following tree harvesting. Yarding operations can disrupt natural drainage pathways and can create new ones, and can result in gullies infilled with woody slash, increasing the potential for or magnitude of debris torrents.
There are three landslide-related issues to be considered. The first issue is the potential for slides to occur in a particular area. This can be assessed on the basis of the number of events that has occurred both in natural settings and in association with forest development activities. The more slide sites there are, the greater the potential for future landsliding events.
The second issue is that of delivery. Delivery is defined as the potential for the slide debris to enter a stream. The highest risk sites are those where there is a continuous slope to the edge of a stream. In these situations, opportunities to prevent a slide from impacting the stream are limited. The least risk sites are those where the stream is separated from the slope by a broad valley flat.
The final issue is the potential for transfer of material down the stream after it has entered a watercourse. Sediment transport, particularly of coarse material, is a function of stream gradient. The steeper the gradient, the more material moved and the greater the distance of travel. Low gradient sections of channel are typically braided (multi-channel) as a result of sediment deposition.
Number of large landslides hitting the mainstem (indicator #11): The mainstem includes the stream reaches with the highest stream order in the sub-basin. In cases where the highest order reach is very short, all channels for a distance of at least 1 km upstream of the POI should be considered “mainstem.” Large landslides are those with a surface area greater than 5,000 m² (e.g., 200 m × 25 m).
Roads on unstable slopes (indicator # 12): Measure the length of roads that cross potentially unstable landscapes, as defined in Appendix D, and divide the number by the total sub-basin area.
Area of unstable slopes logged (indicator #13): Overlay a terrain stability map onto a forest cover map and measure the total area of Class IV and Class V terrain that has been logged. If there are no terrain stability or environmentally sensitive area (ESA) maps for the watershed, contract a professional terrain mapper to map by air photo interpretation all potentially unstable terrain (this is equivalent to Es1 and Es2 mapping). If all else fails, colour all slopes greater than 60% and use this area as a proxy for slope stability information. This is the least desirable option.