This document discusses meandering river systems. It describes how meanders form through the erosion of the outer bank and deposition on the inner bank, creating sinuous bends in the channel over time. Key features of meandering rivers discussed include point bars, natural levees, crevasse splays, and oxbow lakes. The document also covers the helical flow processes responsible for erosion and deposition patterns in meander bends and defines the different facies deposits found in meandering river channels.

1. MEANDERING RIVER SYSTEM

2. CONTENTS
• INTRODUCTION
• MEANDERING RIVER SYSTEM
 FORMATION
 MORPHOLOGY
 PROCESSES
 FEATURES
 FACIES
 CONCLUSION

3. INTRODUCTION

4. MEANDERING STREAM
A meander in general is a bend in a sinuous watercourse

5.  FORMATION
• The thalweg in a river (which is the line of the
deepest part of the channel where there is the
highest velocity flow) is not straight even if the
channel banks are straight and parallel. It will
follow a sinuous path moving from side to side
along the length of the channel. In any part of
the river, the bank closest to the thalweg has
relatively fast flowing water against it while the
opposite bank has slower flowing water
alongside

6. Meanders develop by the erosion of the bank
closest to the thalweg, accompanied by
deposition on the opposite side of the channel
where the flow is sluggish and the bed load can
no longer be carried.
With continued erosion of the outer bank and
deposition of bedload on the inner bank, the
channel develops a bend and meander loops are
formed

8.  MORPHOLOGY
 They are most commonly formed in
coastal plain regions.
 They are characterized by single
channel in contrast to multichannel
braided streams

9.  Its morphology results from disruption in uniform
flow across the channel, caused by variation in;
 Sediments
 Slope or Gradient
 Bed roughness
 The channel profile comprises of a steep side & a
gently sloping side relative to stream bed.
 The steep side experiences lateral erosion and
gently sloping side is characterised by
sedimentation

11.  PROCESSES
 They are characterized by turbulent flow;
velocity varies both horizontally & vertically
across the channel.
 It transport the material both as bed-load
and suspended load.
 Unlike braided streams, meandering
streams provides a regular pattern of flow.
 There is a consensus about the flow in
meanders which may be ellaborated as;

12. Helical flow
 It is the major flow in the meander bends.
 This flow causes an elevation of water level on
the outside of meander.
 This helical flow produces a component of flow
which is normal to stream bank; towards the
eroding bank near the surface and towards the
accreting bank near the bottom
 This has the net effect of producing a
circulation cell which interacts with the bed to
carry sediments upslope along the accretion
surface
HELICAL FLOW

14.  FEATURES
 There are numerous environment &
subenvironments related to meandering
streams; each having characteristics deposits
 These includes;
Channel Lag
Point Bar
Overbank Deposits
Natural Levees
Crevasse-splays
Oxbow Lakes

15. • CHANNEL LAG
Below the channel floor or thalweg,
coarsest material is sorted out and left
behind on the stream bed. Lying just above
the basal erosional surface (or scour base)
is the channel lag which consists of
mudclasts and blocks from bank erosion,
plant debris, boulders and bed-load sand
and gravel

16. • POINT BAR
The sediments accumulating on the convex
side of the meandering loop as the channel
is migrating and the outside bank is eroding
results in the formation of point bar. Most of
the sedimentation in meandering streams
occur in form of point bar

17. Sediment deposited by a river on a valley
flow outside the stream channel. Such
waters usually contain much sediment in
suspension resulting in fine layers of
silt/sand deposition known as overbank
deposits
• OVERBANK DEPOSITS

18. Sheets of sand and silt deposited during floods
are thickest near to the channel bank because
coarser suspended load is dumped quickly by
the floodwaters as soon as they start flowing
away from the channel. Repeated deposition of
sand close to the channel edge leads to the
formation of a levee’, a bank of sediment at the
channel edge which is higher than the level of
the floodplain
• NATURAL LEVEES

19. In times of flooding, the river breaches its
banks. It may temporarily cut through the
outer levee and spill large quantities of
water and sediments. This is known as a
crevasse splay which is typically in a lobe
shape with a mixture of fine and coarse
grained sediments.
• CREVASSE SPLAY

21. Oxbow lakes are created when growing
meanders intersect each other and cut off a
meander loop. These occurs when
meanders grow laterally through erosion
(outside bend) and sediment deposition
(inside bend, point bar). When the loops get
too large and consume too much energy
(friction), the river will eventually find a less
energetically “taxing” shortcut and a part of
the old channel will be abandoned and
becomes an oxbow lake. Over a period of
time, these oxbow lakes tend to dry out or
fill in with sediments.
 OXBOW LAKE

34. MEANDERING RIVER FACIES

35. A facies is a body of rock characterized by a
particular combination of lithology, texture,
suite of sedimentary structures, fossil
content, colour, geometry, paleocurrent
pattern, etc. A facies is produced by one or
several processes operating in a
depositional environment.

36.  Walther's Law of Facies named after the
geologist Johannes Walther(1860-1937),
states that: the vertical succession of facies
reflects lateral changes in environment.
Conversely, it states that when a
depositional environment "migrates"
laterally, sediments of one depositional
environment come to lie on top of another.

37.  In meandering river channels, the facies
always fines upward starting with an erosive
base
 Meandering river channels facies:
 Scoured base of flow
 Channel Lag deposits
 Fining upward sand with trough cross stratification
 Rippled sands
 Cross stratification from migrating point bars

42. CONCLUSION

43. THANK YOU
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