Jr. Goff et P. Ashmore, GRAVEL TRANSPORT AND MORPHOLOGICAL CHANGE IN BRAIDED SUNWAPTA RIVER, ALBERTA, CANADA, Earth surface processes and landforms, 19(3), 1994, pp. 195-212
The relation between morphological change and patterns of variation in
bedload transport rate in braided streams was observed by repeated, d
aily topographic surveys over a 25 day study period in a 60 m reach of
the proglacial Sunwapta River, Alberta, Canada. There are two major p
eriods of morphological change, each lasting several days and each inv
olving the complete destruction and reconstruction of bar complexes. B
ar complex destruction was caused by redirection of the flow and by do
wnstream extension of the confluence scour zone upstream. Reconstructi
on involved accretion of unit bars on bar head, flank and tail and in
one case was initiated by disection of a large, lobate unit bar. High
rates of sediment movement, measured from net scour and fill of the cr
oss-sections, coincided with these morphological changes. Sediment was
supplied from both bed and bank erosion, and patterns and distances o
f transfer were highly variable. Rates of transport estimated by match
ing upstream erosional volumes with downstream deposition were much gr
eater than those estimated from either a step-length approach or a sed
iment budget. Measurements of scour and fill and observations of morph
ological change indicate that step lengths (virtual transport distance
s) were typically 40-100 m during a diurnal discharge cycle. Shorter s
tep lengths occurred when transfer was confined to a single anabranch
and longer steps involved channel changes at the scale of the entire r
each. Sediment budgeting was used to describe the spatial patterns of
sediment transport associated with the morphological changes and to es
timate minimum daily reach-averaged transport rates. Mean bedload tran
sport rates correlate with discharge, but with considerable scatter. T
he largest deviations from the mean relation can be tied to phases of
channel incision, bank erosion, scour hole migration, bar deposition a
nd channel filling apparently controlled by changes and fluctuations i
n sediment supply from upstream, independent of discharge. These are i
nterpreted as field evidence of 'autopulses' or 'macropulses' in bedlo
ad transport, previously observed only in laboratory models of braided
streams.