Meandering Rivers

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Meandering Rivers
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Meandering Rivers

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

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Flow Within Meandering Rivers
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Sedimentation in Meandering Rivers
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Stop Thursday
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Idealized PointBar Sequence
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Lower Point Bar
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Upper Point Bar
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Alteranting climbing ripples and parallel
laminations of an upper point bar in outcrop
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Flood Plain and Levee
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Paleotransport
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Lateral Accretion
Rates 10s to 100s of meters per century
filling channelsthat are on average 3 to 10
meters deep
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Lateral Accretion
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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.
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Alluvial Plains and Meanderbelts
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Stacked channels in Triassic Poleo Sandstone,
New Mexico
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Thick floodplain Deposits
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Base-Level (eustasy) effect on fluvial morphology
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Lowstand Valleys
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Base-Level Control on Rivers (sealevel versus
climate)
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Rivers incise whenShelf gradient gt River
gradient
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The Central Texas Shelf
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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.
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Seismic Stratigraphic Surfaces1. Sequences
Boundaries2. Maximum Flooding Surfaces3.
Transgressive Surfaces
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Seismic and Chronostratigraphy(Oxygen isotopes,
biostratigraphy, radiocarbon dating)
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Systems Tracts(http//gulf.rice.edu
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Fluvial incision during Last Glacial Maximum
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Different Valley Geomorphologies
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The Stage 2 Sequence Boundary
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Headword Erosion(Sea-level falls below shelf
break)
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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)
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3D Geomorphology of the Brazos and Trinity
Valleys High versus low sediment supply fluvial
systems
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Vertical Exaggeration 300
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The Brazos River(Example of an overfed valley)
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The Trinity Valley(example of an underfed valley)

• Re-occupation of valley (fill and purge)
• Terraced morphology
• Point source

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Influence of Antecedent Topography on valley fill
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Antecedent Topography
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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
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A
Isochron X-Section
A
A
A
x 1,250
x 2,530
Modified from Abbott (2001)
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A-A
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Valley contains 20 km3 of fill that is mostly
Holocene in age - Fill and Purge
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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.

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