Fluvial Environments


Ephemeral rivers are dry during a significant part of the year, contrary to perennial rivers.

Floodplains are the areas occupied by river channels, as well as the surrounding, flat (overbank) areas that are subject to flooding.

Discharge is confined to the channel until bankfull discharge is reached; from that point on overbank flow can occur, submerging the entire floodplain.

Photo: Overbank flow with main river channel in centre.


Channel Patterns

Channel patterns (fluvial styles) are commonly classified as:

Braided rivers

Meandering rivers

Straight rivers

Anastomosing rivers

Fluvial style is primarily controlled by specific stream power (W m -2 ) and grain size, but also by bank stability and the amount of bed load.

Specific stream power is defined by:

p=fluid density; Q=discharge; s=slope (gradient); w=channel width

 

Braided Meandering
Straight Anastomosing

 

Mid-channel bar (near) and point-bar (upper left) Outcrop of point-bar
Bars are sandy or gravelly macroforms in channels that are emergent, mostly unvegetated features at low flow stage, and undergo submergence and rapid modification during high discharge. Braided rivers are characterized by a dominance of braid bars; meandering rivers primarily contain point bars; in straight (and most anastomosing) rivers bars are almost absent. Point bars form on inner banks and typically accrete laterally, commonly resulting in lateral-accretion surfaces; mid-channel or braid bars accrete both laterally and downstream. As the channel continues to migrate, the old position of a point bar is preserved topographically as a system of ridge and swales referred to as scroll bars that can be seen in modern flood plains and in ancient sedementary deposits (above).

 

Channel belts consist of channel-bar and channel-fill deposits ; the proportion of the two generally decreases markedly from braided rivers to straight or anastomosing rivers. The geometry of a channel belt (width/thickness ratio) is a function of the channel width and the degree of lateral migration; values are typically much higher for braided systems (>>100) than for straight or anastomosing systems (<25).

 

Residual-channel deposits are predominantly muddy (occasionally organic) deposits that accumulate in an abandoned channel where flow velocities are extremely small or only present during overbank flow. . As the channel migrates, parts of it may become abandoned and left behind as "Oxbow" lakes. These lakes have a characteristic horseshoe shape that mimics a river bend. They become sites for deposition of fine-grained lake sediment and these mud-plugs may form vertical/lateral permeability barriers.
Oxbow-Lake in abandoned meander channel (Nr. Ob River, W. Siberia) Oxbow lake and the Chippewa River. Eau Claire, Wisconsin. Muddy abandonment sediments in Oxbow.

 

Overbank environments are dominated by fine-grained facies (predominantly muds)

Natural-levee deposits are wedges of sediment that form adjacent to the channel, dominated by fine sand and silt exhibiting planar stratification or (climbing) ripple cross stratification.

Crevasse-splay deposits are usually cones of sandy to silty facies with both coarsening-upward and fining-upward successions, and are formed by small, secondary channels during peak flow.

Flood-basin deposits are the most distal facies, consisting entirely of sediments deposited from suspension, and are volumetrically very important (mainly in low-energy fluvial settings).

Overbank Deposits, Mississippi River

A description and analysis of overbank deposits along a portion of the Mississippi River is presented by Farrell,(1987). The sedimentology of two overbank subdeposits (backswamp and levee/splay deposits) is described in detail. The backswamp deposits consist of a basal blue clay unit with leaf layers. This unit is interpreted as having a lacustrine or poorly drained swamp origin. The blue clay unit is overlain by a silty, sandy unit which is interpreted as representing a channel avulsion event. A mudball unit overlays the silty/sandy unit and is probably formed in response to a major flood event associated with the establishment of the new channel belt. The upper unit in this sequence is a rooted clay zone. This final unit is interpreted to represent overbank deposition onto the floodplain and the vertical accretion of a well drained swamp.

The second facies described are the levee/splay deposits. The sedimentological succession here consist of a basal blue clay, a lower sand/silt unit, a rooted clay unit, and an upper sand/silt unit. The rooted clay deposits are interpreted to be backswamp deposits the interfinger with the sand/silt units. Rhythmites are the primary bedding type in this facies. The sedimentary sequence detected in the cravass splays consist of a lower coarsening upward unit, a thick cross laminated middel unit, and a fining upward upper unit with a wormy fabric. This succession is indicative of early prograding cravass splay sediments overlain by the fully developed cravass splay sands, and capped by the abandonment facies from the cravass splay.

The establishment of a channel belt was also documented and divided into four stages. The pre-avulsion stage corresponds to the blue clay units. This is followed by channel avulsion, and then the establishment of an early channel belt. The final stage is the establishment of a mature channel belt with well developed levees. The overbank sub-environments described above are genetically connected to the channel belt stages.

 

Natural levee Crevasse-Splay
Channel down-cutting into abandonment/overbank mudstones.  

Coal in Fluvial Systems

The presence of significant coal measures in a fluvial system is obviously dependent on first order climatic controls and second order biological controls. The development of the coal is also dependent on channel stability and the development of contained clastic deposition. Many descriptions of anastomosed fuvial systems note the presence of large stable floodplain areas (Miall, 1987). An active fluvial system that is characterized by frequent cravassing or levee overbank deposition will cause the flood plain to be inundated with significant amounts of detrital clastic material. This in turn will result in the development of low quality coals. Higher quality coals may require conditions where rapid accumulations of peat exceeds clastic deposition ratesin the channels and causes the developmend of slightly raised swamps which constrain channel migration (Flores, 1984; McCabe,, 1984).

Coal distributions may also be the result of dendritic drainage patterns. This results in coal seam interruption by the development of channels which are draining the paleo swamps (Padgett and Ehrlich, 1978).

Abrupt transition from overbank, coaly shales to coarse, channel sand  

Paleosols (well drained conditions) and peats (poorly drained conditions) occur frequently in overbank environments and are important indicators of variations of clastic aggradation rates and the position relative to active channels. Lacustrine deposits can be important in overbank environments characterized by high water tables, and are also found in distal settings.


Fluvial Architecture and Facies Successions

Facies successions in sandy to gravelly channel deposits typically fine upward, from a coarse channel lag, through large-scale to small-scale cross stratified sets (commonly with decreasing set height), and finally overlain by muddy overbank deposits. Facies successions produced by different fluvial styles can be extremely similar. The geometry and three-dimensional arrangement of architectural elements therefore provides a much better means of inferring fluvial styles from the sedimentary record.

See CHANNEL-BELT WIDTH PREDICTION AND CONNECTED SANDBODY VOLUMES FROM PRESERVED SANDSTONE THICKNESS IN WELLS for a discussion on Fluvial Channel Architecture and sand-body geometries.

Alluvial architecture refers to the three-dimensional arrangement of channel-belt deposits and overbank deposits in a fluvial succession. The nature of alluvial architecture (e.g., the proportion of channel-belt to overbank. deposits) is dependent on fluvial style, aggradation rate, and the frequency of avulsion. Avulsion is the sudden diversion of a channel to a new location on the floodplain, leading to the abandonment of a channel belt and the initiation of a new one.
Low Agradation/High Subsidence High Agradation/Low Subsidence

 


Glossary

Bars

In-channel accumulations of sediment that are often only inundated at bankfull flows. Very important to channel form and function – bars are an important
part of hydraulic roughness, deflect flow, and active migration of bar forms (slow movement over years, can be re-arranged by big floods) is an important component of sediment transport.

Mid-channel bar: common in zones of rapid deposition (rivers overloaded with
coarse bedload), at channel widenings, etc. As these become common they will
split flow into multiple threads.
Alternate bars: side-channel bars formed in straight channels (mobile bed) – a
natural flow/sediment transport instability that will always form: positive
feedbacks from virtually any initial perturbation to a straight, flat-bed channel.
Point bars: bar forms produced by deposition on the inside of meander bends,
critical to meander migration and alluvial stratigraphy
Back-bar chute: high flow channel often formed at top, inside edge, of point bar.

Crevasse-splay

Fan-shaped wedges of coarse sediment deposited downstream of levee breaks during floods.

Floodplain

Level land along the course of a river formed by the deposition of sediment during periodic floods. Floodplains contain such features as levees, backswamps, delta plains, and oxbow lakes. Floodplains may be extensive, such as below the conflux of the Ohio and the Mississippi, where they have a width up to 80 mi (130 km).

Floodplain channels

Smaller channels important in the flooding and draining of the floodplain (and in the distribution of sediment, development of stratigraphy).

Scroll-bar topography

Series of arcuate topographic ribs left behind a migratingmeander loop – related to migrating bar forms and back-bar chutes.

Thalweg

The line defining the lowest points along the length of a river bed or valley.

Depositional Environments: Main