The depositional stratigraphy of within-channel deposits in sandy
braided rivers is dominated by a variety of barforms (both singular
`unit' bars and complex `compound' bars), as well as the infill of
individual channels (herein termed `channel fills'). The deposits of
bars and channel fills define the key components of facies models for
braided rivers and their within-channel heterogeneity, knowledge of
which is important for reservoir characterization. However, few studies
have sought to address the question of whether the deposits of bars and
channel fills can be readily differentiated from each other. This paper
presents the first quantitative study to achieve this aim, using aerial
images of an evolving modern sandy braided river and geophysical imaging
of its subsurface deposits. Aerial photographs taken between 2000 and
2004 document the abandonment and fill of a 1 3 km long, 80 m wide
anabranch channel in the sandy braided South Saskatchewan River, Canada.
Upstream river regulation traps the majority of very fine sediment and
there is little clay (<1%) in the bed sediments. Channel abandonment
was initiated by a series of unit bars that stalled and progressively
blocked the anabranch entrance, together with dune deposition and
stacking at the anabranch entrance and exit. Complete channel
abandonment and subsequent fill of up to 3 m of sediment took
approximately two years. Thirteen kilometres of ground-penetrating radar
surveys, coupled with 18 cores, were obtained over the channel fill and
an adjacent 750 m long, 400 m wide, compound bar, enabling a
quantitative analysis of the channel and bar deposits. Results show
that, in terms of grain-size trends, facies proportions and scale of
deposits, there are only subtle differences between the channel fill and
bar deposits which, therefore, renders them indistinguishable. Thus, it
may be inappropriate to assign different geometric and sedimentological
attributes to channel fill and bar facies in object-based models of
sandy braided river alluvial architecture.