OUTLINE.

Introduction.Origin of fluvial channel and stages.River/channel forms.River/fluvial deposits.• Braided river deposit.• Meandering river deposit.

Economic aspect of fluvial deposits.Conclusion .References.Appreciation.

AIM AND OBJECTIVES.

Our aim is to give a concise and detailed talk on fluvial channels.

Our objectives are within the following;• Origin of fluvial channel.• Fluvial processes.• The forms of fluvial channel.• Fluvial deposit• Their economic aspect.

INTRODUCTION.WHAT IS FLUVIAL CHANNEL?

Fluvial Channel is a term used in Earth science to refer to the pathway through which river flows, the processes associated with river, the deposits and landforms created by them.

Fig. 1, showing a river channel.

WHERE RIVERS ORIGINATE FROM AND STAGES.

Rivers take their source from mountainous regions, after which they flow downstream into lower areas/regions.

Fig. 2, showing the source of the river, the stages and the processes.

From the source of the river, three stages are notable in the river’s course which are:1. Early or young stage.2. Middle or mature stage.3. Late or old stage.

Early Stage: it is characterised by the following;1. This occurs in the mountain regions.2. The velocity or energy of the flow is very high.3. The flow of the river is straight to almost

straight.4. Erosional features dominate.5. There is little or no deposition.

Middle stage: it is characterised by the following;1. The river begins to braid.2. The velocity ranges from moderate to high.3. There is moderate erosion.4. Deposition is also moderate, depositing

coarse-grained sediments.

Late stage: it is characterised by;1. Rivers coalesce2. The velocity of the flow is very low.3. The river begins to meander.4. Depositional features dominate.

RIVER/CHANNEL FORMS.According to Leeder (1999, pg. 311), the channel form of rivers can be described in terms of;• The deviation of the channel from a straight path

(sinuosity),• The number of channels(single or multiple),• The degree of channel subdivision by large bedforms

(bars), and accreting islands around which channel reaches diverge and converge (braiding), and

• More permanent distributive channel subdivision into stationary smaller channels (separated by floodplains) that each contain their own channels and point bars (anastomosing).

In this regard, the main forms of river are; straight, anastomosing, braided and meandering channels.

1. Straight channel:• Channel has low to no sinuosity over its length.• They are rare.• They exist only over short distances.

2. Anastomosing channel:• They consists of multiple, interconnected

channels that are separated by areas of floodplain (Makaske 2001).

Fig. 3, Anastomosing river channel.

3. Braided channel:

• Main channel consists of several channels.• Bars are major depositional feature, which are

often submerged at high flow.• There is successive division and rejoining of

flow around islands.• Braided bar is the depositional feature; which

can be longitudinal, transverse or linguoid bar.

Fig. 4, Main morphological features of a braided river. Deposition of sand and/or gravel occur on the mid channel bar.

4. Meandering channel:

• There is high sinuosity in this channel.• The flow at the outer bank (cut bank) is stronger, which

causes the erosion of this portion; while the cross-flow towards the inner bank is weaker, leading to deposition.

• Point bar is the depositional feature.

Fig. 5, Main morphological features of a meandering river. Deposition occur on the point bar, while erosion occur on the opposite cut bank. Levees form when flood rapidly deposits sediment close to the bank.

RIVER DEPOSITS.

• Accumulation and possible preservation of river channel deposits can occur only if the river changes its position in some way, either by shifting sideways, or if the channel changes position on the floodplain, a process known as avulsion.

• Straight and anastomosing channel deposits are usually not found in stratigraphic record, but deposits of braided and meandering channels are usually preserved.

• Generally, fluvial deposits are fining upward sequence units.

BRAIDED RIVER DEPOSITS.

Fig. 6, Depositional architecture of a braided river: lateral migration of the channel and the abandonment of bars leads to the build-up of channel-fill successions.

The alluvium of braided river systems consists, largely of channel lag gravels, cross-bedded channel bar andbraid bar sands.

A notable example of alluvium deposited by braided channel system is found in the Cambro-Ordovician sand blanket, which stretches from the Atlantic ocean across Sahara and Arabia as far as the Arabian Gulf.

Fig. 7, Physiography and deposits of a braided alluvial channel system. Sedimentation occurs almost entirely in the rapidly shifting complex of channels. Silts are rarely deposited in abandoned channels. A floodplain is absent.

MEANDERING RIVER DEPOSITFig. 8, Depositional architecture of a meandering river: sandstone bodies formed by the lateral migration of the river channel remain isolated when the channel avulses or is cut-off to form an oxbow lake.

Alluvium deposited by meandering fluvial system consist of; scoured intraformational erosion surface bevelled across older alluvium or bedrock, channel lag conglomerate, cross-bedded sand bar deposit, rippled sandstone and overbank mud.A classic example of a meandering river deposit is seen in the Devonian Old Red Sandstone of the North Atlantic Margins of the Appalachians.

Fig. 9, Physiography and deposits of an alluvial flood plain cut by meandering channels. This illustration shows how the lateral migration of a channel generates an upward-fining-grain size profile on its inner convex bank.

Other deposits associated with fluvial system are;• Levees: this is a bank of sediment deposited at

the edge of the channel which is higher than the level of the floodplain. They separate the channel from low-lying flood basin on either side of the alluvial plain.

• Floodplain deposits are generally fine-grained sands, clays, and silts. They are interlaminated, cross laminated, and characteristically desiccation-cracked. Flood basin deposits are often burrowed, frequently pierced by plant roots, and, under suitably waterlogged conditions, may become peatforming swamps and marshes.

• The assemblage of levee, floodplain, and swamp sediments is collectively referred to as an overbank deposit, to distinguish it from the assemblage of channel deposits.

• Crevasse-splay: this occurs when rivers at bankfull scour channels through the levees termed "crevasses“. Lobes of fine sand are deposited where these debouch into the flood basins. These crevasse-splays are similar to, though smaller than, the lobes of deltas.

Fig. 10, showing a crevasse-splay.

ECONOMIC ASPECT OF FLUVIAL DEPOSIT.

Fluvial deposits are of great economic importance for many reasons. They serve; • as aquifers or petroleum reservoirs, because

of their porosity and permeability.• as the hosts for deposits of coal, uranium,

and placer minerals.• as important hosts for placer ores of detrital

heavy minerals notably gold (Bache, 1987).

CONCLUSION.

In conclusion:• Fluvial channels has to do with the pathways

through which river flows. They flow downstream from mountainous region to lowlands.

• The major forms of fluvial channels are: straight, anastomosing, braided, and meandering.

• Braided bar is the deposit of braided channel. • Point bar is the deposit of meandering channel. • Fluvial deposits are of great economic

importance, as stated in the presentation.

REFERENCES.

1. Applied sedimentology by R.C Selley.2. Principles of sedimentology and

stratigraphy (4th edition) by Sam Boggs.3. Sedimentology and Stratigraphy by Gary

Nichols.4. Lecture note by Mr O. A Opatola.

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