Citation | Bird, SA. 1998. Riparian response to the development of a lateral sediment wedge. In: Hogan, D.L., P.J. Tschaplinski, and S. Chatwin (Editors). B.C. Min. For., Res. Br., Victoria, B.C. Land Manage. Handb. No. 41. |
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Organization | FLNRO |
URL | https://www.for.gov.bc.ca/hfd/pubs/docs/Lmh/Lmh41.htm |
Abstract/Description or Keywords | At the scale of a watershed, forest ecosystems operate as a landscape consisting of a mosaic of patches different in shape, type, and function (Forman and Godron 1986). Individual forest patches are both interconnected and interdependent, fundamentally linked by the transfer of water through the system. The riparian area amplifies the significance of these connections, as water, biomass, and sediment are concentrated from within a watershed and either temporally stored or transported directly out of the system. DeBano and Heede (1987) suggest that the soils, geomorphology, and hydrology of the riparian area evolves through a series of aggradation and degradation steps following the establishment of a channel structure such as a log jam. (In steep, forested watersheds on the Queen Charlotte Islands, log jams disrupt sediment transport by creating a local base level that initiates the deposition of a sediment wedge [see, for example, Hogan et. al., this volume].) Those plant species most tolerant of a specific range of flooding disturbance establish along a gradient of decreasing flood frequency and intensity above bankfull stage (see Hupp 1988). Consequently, riparian plant communities are expected to form bands that parallel the stream channel, each occupying a unique elevation above bankfull stage. Field work was conducted in 1991 on a 3.75-ha riparian area in the Gregory Creek watershed, Rennell Sound. Thirty-eight discrete patches of riparian vegetation representing four seral stages were identified: pioneer seral (12 yr), young seral (71 yr), maturing seral (99 yr), and maturing edaphic climax (>250 yr). Ages were determined by coring the oldest living trees found in each seral stage and counting the growth rings. The elevation of each seral stage, relative to bankfull stage, was measured by a series of surveyed transects. Only the elevation of the pioneer seral vegetation was unique, and it was therefore concluded that repeated episodes of flooding behind log jams did not represent the dominant process responsible for the distribution of riparian forest patches in Gregory Creek. However, the ages of the pioneer, young, and maturing seral stages did correspond to episodes of mass wasting documented by Schwab (this volume) that occurred throughout the watershed in 1978, 1917, and 1891. Analysis of log jams in the study site suggested that during these episodes, an increase in discharge and sediment transport forced the channel around existing log jams, out of the channel, and into the riparian area. Patches of vegetation were destroyed as log jams shifted laterally across the valley bottom. Once flow subsided and the sediments stabilized, opportunistic, pioneering riparian vegetation colonized the fresh alluvium deposited in the sediment wedge. Patterns of riparian vegetation are ultimately determined by the spatial and temporal adjustment of the stream channel. In Gregory Creek, the response of the stream channel to mass wasting has created a mosaic of forest patches different in age and successional status, independent (with the exception of the pioneer seral) of their position relative to bankfull stage. |
Information Type | abstract |
Regional Watershed | Coast Region |
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Project status | complete |
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