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Meandering Rivers

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Title: Meandering Rivers


1
Meandering Rivers
2
Meandering Rivers
  • Low gradients
  • High channel width to depth ratios
  • Suspended load dominated
  • Perennial Discharge (humid climate)
  • Wide floodplains

3
Flow Within Meandering Rivers
4
Sedimentation in Meandering Rivers
5
Stop Thursday
6
Idealized PointBar Sequence
7
Lower Point Bar
8
Upper Point Bar
9
Alteranting climbing ripples and parallel
laminations of an upper point bar in outcrop
10
Flood Plain and Levee
11
Paleotransport
12
Lateral Accretion
Rates 10s to 100s of meters per century
filling channelsthat are on average 3 to 10
meters deep
13
Lateral Accretion
14
River Avulsion - when a river diverts from its
established course to follow a new course
(Slingerland and Smith, 2004). Most avulsion
events occur during floods. Also, during log
jams, bank slumping events. However, longer term
processes (such as aggradation and growth faults)
set up avulsion.
15
Alluvial Plains and Meanderbelts
16
Stacked channels in Triassic Poleo Sandstone,
New Mexico
17
Thick floodplain Deposits
18
Base-Level (eustasy) effect on fluvial morphology
19
Lowstand Valleys
20
Base-Level Control on Rivers (sealevel versus
climate)
21
Rivers incise whenShelf gradient gt River
gradient
22
The Central Texas Shelf
23
Case Study-the Late Quaternary Gulf of
MexicoModern Gulf Coast rivers have different
climatic settings,drainage basin sizes and river
gradients (coastal plain widths). During
lowstands these rivers flow across shelves with
different profiles. Subsidence rates are
relatively high compared to most passive margins.
24
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25
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26
Seismic Stratigraphic Surfaces1. Sequences
Boundaries2. Maximum Flooding Surfaces3.
Transgressive Surfaces
27
Seismic and Chronostratigraphy(Oxygen isotopes,
biostratigraphy, radiocarbon dating)
28
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29
Systems Tracts(http//gulf.rice.edu
30
Fluvial incision during Last Glacial Maximum
31
Different Valley Geomorphologies
32
The Stage 2 Sequence Boundary
33
Headword Erosion(Sea-level falls below shelf
break)
34

Incised Valley Classification
  • Furthermore
  • Subdivisions
  • Underfilled (A)
  • Wave dominated
  • Tidal dominated (Gironde Incised Valley)
  • Overfilled (B)
  • Bedload dominated
  • Suspended load dominated

Two type cross sections
A Modified from Dalrymple, Zaitlin, and Boyd
(1994)
35

3D Geomorphology of the Brazos and Trinity
Valleys High versus low sediment supply fluvial
systems
N
Vertical Exaggeration 300
36
The Brazos River(Example of an overfed valley)
37
The Trinity Valley(example of an underfed valley)
  • Re-occupation of valley (fill and purge)
  • Terraced morphology
  • Point source

38
Influence of Antecedent Topography on valley fill
39
Antecedent Topography
40
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41
403 Digital Water Well Descriptions
Influence of sea-level rise on valley aggradation
Clay
Sand
Note the distribution of clay and sand
Vertical Exaggeration 300
42
A
Isochron X-Section
A
A
A
x 1,250
x 2,530
Modified from Abbott (2001)
43
A-A
44
Valley contains 20 km3 of fill that is mostly
Holocene in age - Fill and Purge
45
Stacked Holocene channels of the Brazos River
  • Base of valley contains lowstand fluvial sands.
    Upper part of valley contains isolated channel
    sands encased in fine-grained floodplain
    deposits. The stacking pattern of these sands is
    ultimately determined by sediment supply and the
    rate of sea-level rise.

46
Stop thursday
47
Transgressive Systems Tract
48
Transgressive Fluvial Channels - typically end
abruptly down-dip where they are truncated by the
transgressive ravinement surface
49
The Colorado - example of an incised
transgressive valley(remember, avulsion occurs
mainly during rise in sea level
50
Highstand Valleys
51
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