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Watershed responses to timber harvesting disturbance Campbell, David Andrew

Abstract

In mountainous watersheds, such as those in the Coast Mountains of British Columbia, sediment mobilized on hillslopes can be delivered directly to stream channels. In coupled watersheds timber harvesting operations increases landslide sediment production on hillslopes, and decreases channel stability where riparian logging affects bank strength or sediment supply is increased due to landslide delivery. This report documents an investigation of hillslope and stream channel responses to timber harvesting in 119 reaches of 7 study watersheds in the Coast and Cascade Mountains of British Columbia. Air photos and GIS were used to document landslide and stream channel changes over a 40 to 50 year period within each study basin. Response times, response magnification, and relaxation times were calculated for hillslope and stream channel responses. Five response regimes are proposed to explain watershed response mechanisms; no response, riparian-driven responses, landslide-driven responses, propagated response, and compound responses. Channel change was considered to be significant if width changes exceeded 5.0 m and 7% of the initial width. The 46 reaches that did not exceed these criteria are considered to have no significant response. Riparian-driven responses were observed in 40 reaches, and average channel widening was 94% of initial channel width. Landslide-driven responses were observed in 16 reaches, and average channel widening was 86%. Propagated responses were minor (8 reaches), though they exhibited the largest amount of channel width increase (179%). Channels that exhibited compound responses had characteristics similar to the other three response types, and therefore the mechanism of response was difficult to discern. Overall 17 reaches exhibited compound response, and the average channel width increase was 120%. Average response times ranged from 17.4 to 22.5 years for all channel response types, and response time was 16.4 years for landslide responses Relaxation times were 13.8 years for landslides, 14.2 years for landslide-driven channel responses, and 28.4 years for riparian-driven responses. Confined channels are less prone to significant channel responses than channels with developed alluvial floodplains. An important issue for land management is the potential for disturbances to propagate to undisturbed reaches downstream.

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