Logging-related increases in stream density in a northern California watershed
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Although many sediment budgets estimate the effects of logging, few have considered the potential impact of timber harvesting on stream density. Failure to consider changes in stream density could lead to large errors in the sediment budget, particularly between the allocation of natural and anthropogenic sources of sediment.
This study conducted field surveys in randomly selected catchments in two managed and one old-growth watershed to determine the location of the channel's origins in the catchments. The drainage areas for identified channel heads were then delineated using a 1 m digital elevation model derived from laser altimetry. The two managed watersheds were heavily impacted by previous logging activities, particularly by tractor operations used to yard the timber out of the watersheds. The channel heads in the managed watersheds had smaller drainage areas than channels in a nearby old-growth watershed. The management activities led to a tripling of the drainage density in the managed watersheds.
Timber harvesting and the construction of skid trails used to transport timber to the road system led to increases in peak flow, ground water interception, soil compaction and drainage diversion, which reduced the drainage area necessary to initiate stream channels. Furthermore, it appears that recent ground-based yarding operations have further extended stream channels upslope, potentially creating additional sources of sediment for downstream receptors. Although these results may be unique to these watersheds, the changes in drainage density due to management activities found here emphasize the need to compare managed watersheds with undisturbed watersheds before using the current drainage network as a base-line for estimating chronic sources of sediment like bank erosion.