VOCAB
Facies – Rock formation specific to sedimentation process of the area/the environment
Thalweg – deepest line/main line running through a watercourse. Main stream of water. Continuous.
Bar –“ bedform whose length is proportional to channel width and whose height is comparable with the
mean depth of the formative flow (ASCE, 1966)”
The sedimentology and alluvial architecture of the sandy braided South Saskatchewan
River, Canada (notes)
Cant & Walker (1978) proposed that the South Saskatchewan channel-belt could be
characterized by three different facies profiles (Fig. 1): ‘sand flat’ (profile A), ‘channel’ (profile C)
and a combination of the two termed ‘mixed influence’ (profile B).
‘Sand flats’ can be 50 m to 2 km in length in low floodand have channels running across them.
The key differences between facies profiles A and C (in below diagram by cant and walker) is the
amount of deposition in bars , and the dunes
(from pg 2)
The site
“The sask river is incised by up to 30 m into Cretaceous shales and sandstones and Quaternary
deposits. At Outlook, the channel belt is approximately 0.6 km wide, has an average bed slope of
0.0003 (Cant, 1976) and a braided planform. The braiding index (the number of active main
channels per cross-stream transect at low-flow stage) within the 10 km long study reach is 2.2.”
grain-size ranges from 0.22 to 0.44 mm Clay and gravel are rare.
“Scrub grass vegetation, willow bushes and small trees stabilize bars within the channel and on
the floodplain. The main (or first-order) channels are 2–5 m deep and 50–150 m wide. Cant &
Walker (1978) used echo-sounding to show that the deepest channels are dominated by dunes
that may be up to 1.5 m high during floods, but are more commonly 0.3–0.5 m high.”
Small channels get ice cover in winter, but water flows free below.
Ground penetrating radar
Twelve common midpoint (CMP) surveys allowed calculation of the mean radar wave velocity as
0.051± 0.006 m nsec
The findings are that there are subtle grain size changes (see pg 5) as flow changes.
The evolution of changes in the deposition was analyzed using areal photos over time.
Above shows common bars of the South Saskatchewan River: types - unit and compund.
Unit bars : not amalgamated or changed during migration.
four main shapes of unit bar, in South Sask River the majority of have a lobate planform with a
slipface - called by smith a transverse bar. The highest point is at the downstream end of the
bar, with an “avalance face”
don’t gain much height above the water surface.
“Unit bars experience significant change over time periods of 1 year, with the highest rates of
downstream migration (20–130 m between April and October 2000) being recorded for small unit
bars (e.g. %£ 150 m long) in the largest channels. Unit bars in the South Saskatchewan River
represent an early stage in the development of compound bars.”
compound bars : more than one unit bar evolve through several erosion and deposition events.
Leaving a wider range of platform shapes.
Dunes can be super-imposed on unit and compound bars.
One noticeable form is downstream elongated limbs where one unit bar forms the central core
that then promotes accretion from additional unit bars on either side.
“Compound bars can develop an asymmetric morphology (labelled CBa in Fig. 6), with one bartail
limb becoming longer than the other, that is associated with developing flow asymmetry within the
distributaries (Ash- worth et al., 2000). These newly formed com- pound bars also have little
topographic relief above the water surface, and average bar dimensions are %180 m downstream
and %120 m across-stream. The nature and rate of change for small compound bars are similar
to that recorded for unit bars, but as compound bars evolve and become larger, their origin from
the formative discrete unit bars becomes less clear; they may no longer retain the downstream
limbs in their morphology (labelled CB in Fig. 6) and their rate of migration slows. Compound bars
grow when either other bars migrate and stall on their head/margins or the channel separating a
compound bar from an adjacent bar becomes abandoned and fills, resulting in creation of a
single, much larger bar”
Quantification of the occurrence of facies within different types of braid bar
“So as to establish the frequency of occurrence of the different facies throughout the 3.5 km of
GPR survey lines, the following methodology was used. Individual survey lines for each bar were
first interpreted based on the facies types described above. A vertical section was then sampled
every 10m along each line, thus allowing the proportion of each facies within each vertical section
to be determined for each survey line. By combining data from all the vertical sections, the overall
facies proportions for each bar were produced (Table 3). Additionally, so as to quantify the
vertical variability of facies within each bar, every vertical section was also divided into 0.5 m
intervals measured upwards from a clear basal erosion surface present within the GPR surveys.
Topographic surveys of the major channels indicate that this surface is at a depth similar to that
of the modern river thalweg. The erosion surface in the GPR thus represents the channel base
above which bars initially develop and so identifies the likely maximum bar depth. The proportion
of each facies within each 0.5 m vertical interval was then determined and summed, thus
enabling the vertical distribution of facies to be quantified. The total number of vertical sections
sampled and facies proportions present for each bar are given in Table 3.” (pg16)
“All three profiles given in the model of Cant & Walker (1978; Fig. 1) show high-angle inclined
strata associated with unit bars (the cross-channel bars of Cant & Walker). However, given that it
has been shown herein that unit bars do not preserve 1–2 m thick sets of high-angle inclined
strata, this facies (facies 1 herein) is probably over-represented in the model of Cant & Walker
(1978). “
CONCLUSION
6 conclusions:
1. Dominent facies are large-scale inclined strata formed by migration of bar mar- gins; mediumand small-scale cross-stratification formed by sinuous crested dunes and ripples; low-angle strata
formed by the migration of dunes or unit bars; cross-bar channel fills.
2 Higher up in the profile, cross-strata associated with smaller dunes becomes more common
with low-angle strata dominating the deposits near the bar surface. High-angle inclined strata are
only found near the surface of unit bars, and are relatively small-scale, but can be found at a
wider range of depths in compound bars.
3 Unit bars are dominated by cross-stratification formed by dune migration (60% of facies 2),
which may merge laterally to form high-angle inclined strata. Cross-stratification associated with
dunes decreases in size from thalweg to bartop, reflecting the decreasing formative flow depths.
Unit bars possess only a minor amount of high-angle inclined strata (< 10% of facies 1), generally
found at bar margins.
4 Flow across the top of a compound bar generates high-angle cross-stratification (facies 1) at
the bar margin, whereas if a compound bar does not experience overbar flow, then low-angle
planar stratification (facies 3) will predominate the resultant facies in the middle-upper parts of the
profile.
5 The largest compound bars comprise an amalgamation of other smaller bars (both com- pound
and unit), and their facies distribution is thus similar to the average for all other bars.
6 The features described herein for the South Saskatchewan River possess similarities to many
other modern and ancient sandy braided rivers described in the literature. However, the GPR
data demonstrate a high variability in facies both within and between bars, with this variability
being greatest within compound bars.
The present study suggests there is a need for more GPR studies from a range of other braided
rivers to help elucidate the key relationships between formative conditions and alluvial
architecture, assess if a single generic facies model can be applied to all braided rivers, or
conversely, if a range of models is required. Future work should focus on establishing a direct link
between the processes operating in braided rivers, the changing morphology through successive
flood events and the resultant sub-surface alluvial architecture across the entire braidplain.