Water Stewardship Information Sources
| Citation | Millard, T. 2000. Channel disturbance and logging slash in S5 and S6 streams. Forest Research Technical Report TR-005, Vancouver Forest Region. |
|---|---|
| Organization | FLNRO |
| URL | https://www.for.gov.bc.ca/rco/research/georeports/tr005.pdf |
| Abstract/Description or Keywords | The Forest Practices Code of BC allows logging of some S5 and S6 streams (non-fish bearing streams that are not in community watersheds) without leaving a streamside reserve. During silviculture prescription development, if the decision is made to not leave a streamside reserve, then an assessment of whether to recommend removal of logging slash from the stream may be needed. An important factor in determining whether logging slash should be removed from the stream (often called stream cleaning) is whether it will lead to increased volumes of sediment or woody debris transported into downstream channel reaches, particularly fish habitat. This study examined logged and uncleaned S5 and S6 streams in the Nitinat Lake area of Vancouver Island to assess the degree of channel disturbance that resulted from leaving logging slash in streams. Channels that have higher levels of disturbance are probably transporting elevated amounts of sediment and logging debris into downstream channel reaches, whereas channels that are not disturbed are not transporting elevated amounts of sediment and logging debris downstream. Ninety-nine stream reaches were field assessed. Data were collected for stream location, terrain type, channel width, channel depth, channel area, gradient, bank, and bed type, and the maximum size of sediment moved in the channel. How the channel was affected by logging slash was evaluated using several specific response parameters: · the size of logging slash transported by water · the percentage of logging slash moving in the stream · the size of logging debris jams · the percentage of logging debris incorporated into debris jams · the size of sediment wedges, and · the amount of bed and bank erosion. In addition to the specific response parameters, each stream reach was given an overall channel disturbance assessment, based on the presence and degree of one or more of the specific response parameters. The five classes of overall disturbance were: no disturbance, little disturbance, moderate disturbance, extensive disturbance, or severe disturbance. There were no predetermined criteria that equated set levels of specific responses to a particular overall disturbance level, and there was no requirement that any specific response must be part of the overall disturbance assessment. Therefore, part of the analysis compared the overall disturbance with each of the specific responses to determine which specific responses were strongly associated with the overall disturbance. Overall disturbance was found to be associated with the size of logging debris moving, the percentage of logging debris moving, the size of logging debris jams, and the size of sediment wedges. Bank and bed erosion were not associated with overall disturbance level. Most streams (64%) in the study had little or no disturbance. Twenty percent of the streams were moderately disturbed, 13% extensively disturbed, and 2% severely disturbed. The results of this study showed that overall disturbance was strongly dependent upon the size of the channel. Whether channel size was measured using width, depth, or area, the level of disturbance increased as the channel size increased. Width was statistically the most significant of these three variables, and therefore is the most useful channel parameter to identify the likelihood of a channel to become disturbed if logging slash is left in the channel. All streams in the study less than 1.5 m wide had little or no disturbance, whereas 90% of streams greater than 3.0 m had moderate or worse disturbance. Eighty-two percent of streams between 1.5 and 3.0 m wide had little or no disturbance. Only 15% of the channels in this study had either bed or bank erosion. Bank erosion tended to increase as channel width increased, but bed erosion was not associated with any channel characteristics. In addition to channel width, the size of sediment a channel transports, and the size of woody debris a channel transports, can be used as predictors of the likelihood of a channel to become disturbed if logging slash is left in the channel. There was usually little or no channel disturbance in streams that transported sediment <90 mm in diameter, whereas channels that transported sediment >250 mm most often had extensive or severe disturbance. If a channel transports woody debris of a particular size after logging (as was measured in this study), it is likely able to transport woody debris of that size prior to logging. Therefore, the size of woody debris transported in an unlogged stream is a useful predictor of whether the channel is likely to become disturbed after logging. Streams that transported small woody debris usually had little or no disturbance, whereas streams that transported larger woody debris were more likely to become disturbed. |
| Information Type | report |
| Regional Watershed | Vancouver Island South |
| Sub-watershed if known | |
| Aquifer # | |
| Comments | |
| Project status | complete |
| Contact Name | Tom Millard |
| Contact Email | [email protected] |