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Long-term influence of jams and LWD pieces on channel morphology, Carnation Creek, B.C. Luzi, David S.

Abstract

The importance of large woody debris (LWD) pieces and jams on channel morphology and aquatic habitat is reflected by the growth in research in this area that has occurred in the last thirty years, especially in the Pacific Northwest. Despite a long history of research, relatively little attention has been paid to the spatial and temporal effects of jams on channel morphology. Twenty-eight years of cross-sectional surveys (1971-1998) and two extensive longitudinal profile surveys (1991 and 1999) document the influence that jams have on channel morphology at Carnation Creek, B.C. Dramatic changes were observed upstream of a recently formed jam, which included bankfull width increases as much as 178% and sediment accumulation that resulted in decreased mean depths and, in some cases in-channel elevation exceeding bank elevation by 0.5 m. Jam-related changes in channel morphology were found to occur throughout the longitudinal profile of the stream. Variation of LWD volumes, bankfull width, stream gradient, sediment size and in-channel sediment storage were found to increase with proximity to jams. This variation not only depends on jam presence and position within the channel but also on jam age, with younger jams having the greatest influence on these morphologic parameters. A novel approach using cumulative departure plots successfully identified zones of aggradation and degradation; these zones were in large part determined by jam functioning in each of the zones. LWD characteristics between the two longitudinal profile surveys changed, reflecting the trend to larger size classes in LWD diameter and length. Changes in jam characteristics as a ' result of jam aging between the two surveys periods were identified. A period of jam 'conditioning' was identified as being an essential determinant in the overall influence of a jam on channel morphology. As a result of the aging process jams have a tendency towards decreased influence on channel morphologic parameters, which is primarily due to a reduction in sediment retention ability with time. LWD was present in at least 88 % of the pools in Carnation Creek, with jams being proximate to greater than 65 % of the pools. An analysis of residual depths was undertaken to examine variation in channel thalweg elevation. Variation between 1991 and 1999 was found to occur at smaller scales than previously reported. This study further elucidates the spatial and temporal co-evolution of LWD jams and channel morphology. Inferring channel forming events based on the ages of jam structures may prove to be a useful tool when attempting to design forest road crossings in areas with inadequate peak flow records. An understanding of the spatial and temporal nature of jams may also aid in the design and implementation of in-stream restoration projects in anthropogenically and naturally disturbed systems.

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