Meandering Channel Systems
Point Bar Development
Gravelly Meander Lobes
Examples of Modern Meandering River Systems
In contrast to braided rivers, meandering rivers typically only contain one channel that winds its way across the floodplain. As it flows, it deposits sediment on banks that lie on the insides of curves (point bar deposits), and erode the banks on the outside of curves. When the river floods, it deposits fine-grained material on the floodplain. As the floodplain is a lot larger than the channel, deposits of meandering river systems are dominated by fine-grained material; coarse-grained channel deposits tend to be relatively minor.
|Click here to see examples of Meandering River facies in cores from the Cretaceous of British Columbia (from University of British Columbia, Canada)|
Meandering channel systems are favored by conditions of a low braiding parameter (<1) and high sinuosity (>1.5). Slopes are low and width/depth ratios of less than 40 are expected. Additionally meandering channels are favored by a predominance of suspended load and cohesive banks. In a generalized model of a meandering river the meanders migrate by undercutting the bank on the outside of a bend and depositing on a point bar on the inside of a bend. The point bar grows through lateral accretion and is characterized by a fining upward sequence. As the flow enters a bend a helical overturn flow develops (diagram) which directs the bottom current to move up along the slope of the point bar. This flow sorts the sediments with the finest sediments being deposited at the top of the point bar. The lateral accretion surfaces on the point bar form large scale, low angle cross-bedding termed epsilon cross-stratification (ECS), with dips ranging from approximately 1 degree in large rivers to 25 degrees in small rivers. Thickness of the crossbed package is roughly equal to channel thickness. Minor bedforms on the point bar will be oriented downstream, perpendicular to sub-perpendicular to the ECS dip (Miall, 1982).
A complete fining upward sequence is only developed in the downstream portion of a point bar. The upstream portions will tend to have coarser upper facies. Point bars may be modified by chute or neck cutoffs which are channels the develop during high flow stages and cut across the point bar. As meander loops widen and merge the chutes may develop into full neck cutoffs resulting in the development of an oxbow lake.
Crevasse splays develop when levees are topped during high flow. The sedimentary sequence detected in the crevasse splays consist of a lower coarsening upward unit, a thick cross laminated middle unit, and a fining upward upper unit with a wormy fabric (diagram). This succession is indicative of early prograding crevasse splay sediments overlain by the fully developed crevasse splay sands, and capped by the abandonment facies from the crevasse splay (Farrell, 1987). Floodplain deposits are predominantly fine grained, and may be coal bearing if developed in humid environments. Pedogenic carbonate concretions may develop in arid or semi-arid environments due to a leaching process.
Jackson (1978) has subdivided meandering rivers into five separate categories:
ECS (epsilon cross-stratification): ECS is commonly reported in ancient point bar deposits, however many modern studies have failed to detect its presence. Studies by Smith (1987) indicate that ECS is also present in tidally influenced modern deposits but due to difficulties in studying these deposits they have not been detected as often. He proposes a three fold classification of point bar deposits: "(1) Fluvial sandy point bar facies, (2) low-energy fluvial and microtidally-influenced (upper estuary) point bar facies, (3) mesotidally influenced point bar facies deposited in upper and middle estuary settings". Facies corresponding to both of the estuarine influenced facies are similar to those reported in ancient settings where ECS are noted in the meandering river point bar deposits. Smith has listed the following sedimentological and stratigraphic trends of ECS with mud interbeds:
The recognition of fluvial point bars may be aided by the identification of LAS (lateral accretion surfaces). These are the bounding surfaces in ECS units and generally represent the topographic surface of the bar at a point in time. LAS can be either surfaces of erosion or deposition. LAS can often be identified texturally by the paucity of coarse silt, due to their eolian origin (Shepherd, 1987). This would imply high variation in flow rates with periods of low flow and point bar exposure.
Detailed facies studies of a gravelly meander lobe by Campbell and Hendry, (1987) indicate that the basal facies consist of crudely stratified pebble and cobble gravels with minor sands. In this unit the lateral accretion bedding (LAB) dips towards the paleo channel at approximately 5 degrees. This facies forms the gravelly point bar platform which is overlain by discontinuous lenses of sands and gravelly sands. This second facies exhibits trough and planar cross bedding developed through the infill of chute and slough channels during the waning stages of flow. The third faced is interbedded sands, silts and clays which dip as much as 12 degrees towards the paleo channel. This third facies represents lateral accretion along an inner bank, indicating innerbank scour during high flow stage and subsequent lateral accretion during the falling stage. The fourth facies is layered silt, clay and fine sand, and represents flood plain deposition. A note worthy point about this assemblage is that the vertical sequence bears many resemblance's to braided river deposits and could be indistinguishabel from them with limited control points. Detailed studies, however indicate that the deposit is the result of point bar deposition from migration of a stream with only one channel.
Examples of Modern Meandering River Systems
|Sacramento River, CA.||Owens River, CA. Note the different generations of old meanders.||Meandering River in northern Owens Valley, CA|
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.
|Oxbow lake and the Chippewa River. Eau Claire, Wisconsin.||Ob River, West Siberia|