Water Stewardship Information Sources

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.
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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