The increase in bedload and woody debris delivery to streams from mass wasting is probably the most important factor in creating stream channel change following logging.
Increased landslide rates are attributed to many factors. Road building disrupts and concentrates 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 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 mass wasting 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.
Roads on unstable slopes (indicator #12): Measure the length of roads that cross potentially unstable landscapes as defined in Appendix 5, and divide the number by the total sub-basin area.
Streams whose banks have been logged and are on slopes > 60% (indicator #13): Measure the length of all streams in the sub-basin that are on slopes greater than 60% and that have been logged right to the edge without a buffer and divide this number by the total sub-basin area.