The small size of individual headwater streams can be misleading as to their relative importance as water sources. Since they can account for more than 70% of the total stream length, their role, particularly during high runoff conditions, is highly significant.
Most headwater streams have steep gradients and (particularly when incised in V-notch gullies) are a significant source of sediment and organic debris. The principal mechanism for the movement of this material is the debris flow (also called debris torrent).
Water flows in headwater streams can be influenced by road construction and logging. Roads intercept subsurface water and ditchlines act as new stream channels, speeding up the flow of water and its delivery to stream channels. Roads can also divert water from one headwater stream to another, increasing flows many fold in the recipient stream. In situations where headwater streams have multiple road crossings, the influence can be further magnified. Drainage structures designed and installed on lower roads can become overloaded or washed out by increased flows from upstream crossings.
Forest roads can have an influence beyond the road rights-of-way. An example is a switchbacking road on slopes between headwater streams. Intercepted subsurface and surface flows can be delivered overland and into the headwater streams.
Forest harvesting affects snow accumulation and melt, leading to alterations in the rate and timing of water delivery to headwater streams (see references in Appendix 5 on stream flows). This effect can be significant since it is possible to log a high proportion of a headwater stream watershed (that is, watershed rate-of-cut constraints are not placed on first-order drainages, so it is possible that a high proportion of a headwater stream watershed may be within one logging block).
Forestry activities can have a significant influence on the level of sediment input to headwater streams. Both roads and logging can be of concern and are discussed in Appendix 6 and Appendix 8 on surface erosion and landsliding. Headwater streams, however, are of specific concern because they are generally the first channels to be affected. For example, debris torrents occur in V-notch gullies that are headwater streams. Erosion around culverts on hillside streams is a direct impact on headwater streams.
Erosional impacts obscured at the watershed level can be very significant to individual headwater streams. Because of the number of headwater streams, the potential for cumulative effects can be high.
Number of stream crossings on steep slopes: Count all road and stream crossings that are on the mapped steep slopes and shown on the 1:20 000 forest cover map.