Fluvial Channels

A channel can be described by its depth, depth/width ratio and its sinuosity. The general shape of a channel in cross section will be a concave-up erosional base, often with a

conglomeratic lag (Miall, 1985). The sedimentary channel fill is often of fining upward unit containing sand and sandstone facies with structures like i) massive or planar parallel lamination, formed from high velocity flows; ii) through cross-stratification, from migration of dunes; iii) planar cross-stratification, from migration of simple bars; and iv) ripple lamination due to waning flow. Large channels can have a multistorey fill including several erosional surfaces with sand and/or mud fill.

Though a general description can be given of a channel succession, fluvial channel units occur in different settings with different morphology and characteristics. The fluvial system comprises four types of main channels of different styles: i) braided, ii) meandering, iii) straight and iv) anastomosing, depending on grain size, total amount of transported material, proportion between bed load and suspended load, slope gradient, water discharge, discharge pattern and weather it is ephemeral or perennial (Bridge, 2003). Straight and anastomosing channel morphologies are rather uncommon and will not be discussed further here.

2.6.3.1 Braided Channels

Braiding of river channels normally occurs in coarse-grained to sandy bedload rivers of low sinuosity with weak non-cohesive banks, or when streams have variable discharge e.g. in connection with seasonal discharge or ephemeral flows. As bedload accumulates on the channel floor, longitudinal bars develop, thussplit and redirect the river (Collinson, 1996;

Miall, 1977). Different types of bars can develop in the downward accretionary- or lateral accretionary direction, generally with related cross-stratification in the accretionary direction, thus a bar can have a complex development of deposition and erosion and sets of cross-stratification obliquely to each other. The channel geometry of a braided river may be rather sheet-like as the width/depth ratio of these can be very large (Miall, 1985). Braided river systems do not normally have large floodplain areas associated with them, as the channel is not very stable and migrates laterally quite frequently. Nevertheless, these rivers carry a large amount of suspended load, particularly during flood stages. During extensive overbank flooding fine-grained material can be deposited above abandoned channel sandstone strata of the alluvial plain. In cases where the braided streams are due to variation in water discharge and not to grain size, floodplain deposits may be more common and extensive (Miall, 1977).

Bridge (2003) disagreed with Miall (1977) regarding the mechanism of formation of floodplains and stated that “floodplains develop independent of channel pattern and in all alluvial valleys, on alluvial fans and on deltas”.

Deposits from braided rivers are expected to be rather coarse-grained with structures like cross-stratification from bar development and migration, imbrication of pebbles and stones and a lack of large finely-grained floodplain deposits and channel margins, in addition to rather high width/depth ratio of channel and resulting deposits (Miall, 1996).

2.6.3.2 Meandering Channels

The meandering river system is characterized by high amounts of suspended load, as well as sandy bedload (though possible some gravel). Meandering rivers flow with a high sinuosity in discrete belts on low gradient alluvial plains (Collinson, 1996; Emery and Myers, 1996).

The meandering channel migrates within its belt by eroding its outer bank and depositing on its inner bank. The channel is rather stable as it commonly is surrounded by cohesive fine-grained material with well developed levees and thus do not braid easily. As the sinuosity increases channel avulsion will eventually occur, leaving an abandoned channel to be an oxbow lake. During flood or high discharge fine-grained material may be deposited on the floodplain, or levees may be broken creating crevasse channel and splays in which both can initiate avulsion (Collinson, 1996). Point bars are lateral accretionary bars associated with high sinuous channels and are discussed further below.

2.7 Bars

Bars in the fluvial environment are depositional forms, or architectural elements, which develop in different sizes on the channel floor or along the sides of the channel. According to geometry and position relative to the channel segment, bars are variously named as e.g.

longitudinal-, linguoid-, transverse- lateral- and point bars. Miall (1977) suggested a simple classification of three types of bars of e.g. different geometry, grain-sizes, occurrence and internal structure (Table 2.1).

Table 2.1: Bar types and their characteristics. From Miall (1977).

Bar

Type Litholog

y Internal

Structure Height Length Bedform

Rank Common

Large scale inclined strata can be associated with downward accretionary of bars in braided or unbraided rivers and can be recognized on a large scale by vertical changes in grain-size and sedimentary structures, where fining upward is a general trend and ripples and dunes are commonly superimposed bar strata. Tidal influenced bars can often be confused with intertidal-flats or coastal bays, as its upper part often contains relative large amounts of finer material (Bridge, 2003). An illustration of a simple braided channel with a downward accretionary bar and channel switching is shown in Fig. 2.7. The downstream portion of the bar has higher preservation value than the stoss side.

Fig. 2.7: Geometry of a downward accretionary bar with different cross-sections shown. From Bridge (2003)