meandering fluvial systems

Photo by W. W. Little

Meandering Fluvial System(single channel/mud- or

sand-dominated)

Meandering fluvial systems have a single, relatively deep major channel with a sinuosity of at least 1.7 (often greater than 2.5), a low bed/suspended load ratio, and cohesive bank material. Meandering river deposits can be dominated by either sand or by mud and are characteristic of high-discharge perennial rivers flowing over low gradients.


• Low gradient relative to discharge• High discharge relative to load• Low bed load relative to suspended load• Cohesive bank materials (commonly muddy & vegetated)• Perennial discharge with relatively low fluctuation

Conditions Favoring Meandering

Simplified Model

Meandering fluvial systems are typically subdivided into four major facies associations, active channel, abandoned channel, overbank, and splay.

Common Channel Facies

Meandering fluvial systems typically consist of a relatively thin, structureless gravel and trough to ripple cross-bedded sand within channels and laminated mud with thin beds of trough- and ripple-bedded sand on the flood plain. Climbing ripples and flaser bedding can also be present, particularly on point bar tops and levees.

Active vs. Abandoned Channel Fill

Upon avulsion, a channel becomes abandoned and, rather than continued migration of point bars, the channel becomes filled with fine-grained sediment during flooding events.


Active Channel Lag Gravel

The coarsest sediment in a meandering stream is transported along the channel floor. This can be a true gravel or consist of mud intraclasts derived from cut-bank caving.


Trough Cross-bedded Sand

Trough cross-bedded sand as sand sweeps as waves around the point bar. Decreasing velocity results in structures that thin upward.


Planar cross- and Horizontally-bedded Sand

Planar cross-bedded sand forms by deposition off a bar front; whereas, horizontally-bedded sand represents bar top sedimentation.


Bioturbated Mud

Flood plain deposits commonly become bioturbated through burrowing and root growth.

Facies Associations

Meandering fluvial systems are dominated by four major facies associations related to active channel fill, abandoned channel fill, overbank deposition, and crevasse-splay development.

Common Channel Facies

Meandering fluvial systems typically consist of a relatively thin, structureless gravel and trough to ripple cross-bedded sand within channels and laminated mud with thin beds of trough- and ripple-bedded sand on the flood plain. Climbing ripples and flaser bedding can also be present, particularly on point bar tops and levees.

Channel Elements


Point Bars

Point bars are channel elements found mostly on the inside of meander bends. They are formed primarily by lateral accretion, consist of a fining-upward succession of trough cross-stratified sand capped by ripple-bedded sand and overly channel floor gravel.


Lateral Accretion Deposits

Lateral accretion occurs in point bars, mostly during flooding, as a channel migrates in association with cut bank erosion. Minor erosion or settling of fine sediment on the sloping surface of the point bar takes place during low flow stage, creating a series of surfaces that dip toward the channel floor.

Lateral Accretion & Facies Assoc.

Facies prograde laterally as channels migrate but maintain a similar vertical succession related to water depth.


Scroll Bars

Scroll bars are the surface expression of lateral accretion deposits.

Side-channel Bars

Mid-channel Bars



Abandoned Channel Fill (channel plugs)

Avulsion leads to abandonment of a channel. Once abandoned, the channel becomes filled with fine-grained sediment as a clay “plug.”


Chute Channels

Floodplain

Levee

Point Bar

Levee

Point Bar

Bar-Top Facies

Point bars are typically capped by fine-grained levee and floodplain deposits. Levees are often dominated by climbing ripples and flaser bedding in fine sand to silt; whereas, overbank sediment consists of laminated silt and clay.


Overbank Facies


Paleosols

Following flooding, sediment on the floodplain is subjected to atmospheric weathering. The longer the period between flooding events, the thicker and better developed the soil; therefore, paleosols tend to become fewer in number but more highly developed away from the channel.

Crevasse-splay Complexes

Flooding can result in levee breaks (crevasses), leading to delta-like deposition on the flood plain (splays). Splays fan out from the crevasse and can consist of simple lenses of sand to complex multiple-channeled complexes, depending upon the degree and length of flooding.

Geomorphology

Meandering fluvial systems consist of single, sinuous, typically sand-dominated channels surrounded by broad, flat, mud-dominated floodplains.

Channel Belts

As meandering rivers migrate laterally across their floodplain, they produce a channel belt that is preserved as a sheet of sandstone encased within mudstone.

Sand:mud ratiosSand body connectivitySand body geometry (lense vs. sheet)Accommodation space and Preservation potential

Transgressive deposits

Lowstand deposit

Early (slow) rise deposits

Moderate risedeposits

Rapid rise deposits

Highstand deposits

Coastal depositsTidally-influenced fluvial deposits

B

Architecture and Subsidence Rate

Low to high sand:mud ratiosLow to moderate sand body connectivitySand body geometry lens to discontinuous sheetModerate to high accommodation and preservation potential

Large-scale Architecture

meandering fluvial systems

Science

little abandoned channel

little point bars

little paleosols

midchannel bars photo

little overbank facies

little scroll bars

channel floor

little planar cross