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The role of wood in headwater channels and short-term channel responses to harvesting of second growth riparian forests in Southwestern British Columbia Winfield, Nicholas A.

Abstract

A watershed-scale experiment was set up at Malcolm Knapp Research Forest in southwestern British Columbia to study the effectiveness of variable width riparian reserves on physical and biological components of small streams. One component of the experiment was an investigation into the influence of large woody debris (LWD) on the morphology of 12 headwater streams. Pools were selected as a key response variable as they are considered a limiting factor in small streams and serve an important role as a low-flow refuge habitat for fish. All pool forming LWD was inventoried, and pool volumes and percentage of fine sediments in pools were measured before logging (summer 1998) and after logging (summer 1999). Subwatershed pairs were determined based on known and predicted peak stream powers. This enabled within-group analysis of channel response to different riparian treatments. Detailed channel assessments and surveys were undertaken in consecutive years to monitor channel morphology changes over time. LWD was the dominant pool-forming element for all streams regardless of channel type. Linear regression analysis found that the variance in pool volume explained by LWD in three step-pool streams and one riffle-pool stream ranged from 15-50%. LWD did not explain the variance in pool volume in the remaining five perennial streams, including two riffle-pool reaches and three step-pool reaches. Removal of old growth trees prior to the 1920s likely explains the low volume of wood and corresponding low pool volumes in these two riffle-pool channels where LWD is expected to play an important role in pool formation. Target levels of LWD for smaller size classes of fish-bearing streams were developed based on relationships between LWD volume and pool volume and predicted pool spacing for different channel types. Comparison of channel response to riparian harvesting based on peak stream power found that streams with low stream power with clear-cut treatments showed reductions in pool volume, specifically reductions in pool size. These streams had insufficient stream power to remove sediment and debris deposited during logging as compared to streams with high stream power which were able to remove sediments.

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