Tutorial:River Deposits
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[edit] River deposition
The characteristics of water-laid deposits depend on the parent material, the distance and manner in which the material has been moved and the energy conditions in which they have been deposited. However, river sediments (alluvial) tend to be well sorted and individual particles may be rounded and polished by abrasion.
Floodplains are simply the flat areas adjacent to rivers liable to flooding (Brown, 1997). However, the processes of flood deposition, river channel erosion and migration, and slope deposits along the valley margins can produce a complicated series of deposits and landforms. Floodplains are important archaeologically as they have been a focus of human activity providing food, transport, power and fertile agricultural soils. Their archaeological potential is further enhanced by deep burial and waterlogging, which produce ideal conditions for preservation.
Two broad groups of deposits can be identified: overbank and channel deposits.
[edit] Overbank deposits
Are deposited during periods of flooding and result in the vertical up-building of the floodplain. The particle size distribution and rate of sedimentation will depend on suspended sediment load, flood frequency, and location within the floodplain. Deposits include levees, splays and overbank fines.
Levees are ridges that run parallel to the river channel. When a river overtops its banks the water rapidly loses energy and coarser material (usually sand and brash) is deposited along the bank, this accumulates to form levees. When levees are breached and water spills out onto the floodplain, sand can be deposited in a fan across the floodplain surface forming a crevasse splay.
Overbank fines represent the vertical accretion of fine sand, silt and clay from overbank flows. In the lower energy conditions away from the channel, and particularly in backswamps behind levees, even very fine material settles out of the floodwater. As sediment settles out fine sands and silts are deposited before clays, this can produce very fine banding (flood couplets) within the floodplain deposits.
[edit] Channel deposits
Are deposited within and immediately adjacent to the channel itself. Channel migration means that such deposits are not confined to the present day channel location but may be found across the floodplain. These deposits form by a process of lateral accretion and their distribution and nature depends on the stability of the channel, the nature of the bedload, and the rivers flood regime.
Channel deposition can include coarse lag deposits representing the bed load of the channel. Ripple marks may sometimes be preserved in sandy river beds providing information about the current strength and flow depth.
Channel bars usually consist of sand and gravel though they may grade to sand and silt at their edges. They form in areas of slower flow on the inner side of meanders (point bars), where flow diverges (mid-channel bars), and along river banks where the flow is diverted by pool and riffle development (alternating bars).
As channels are abandoned, either through avulsion during floods, or through the formation of point bars and eventual cut-off of meanders, the old river channel (oxbow lake) will then infill. Lag and bedload deposits will be overlain by finer deposits deposited during periods of overbank flooding. During periods of flooding old river channels can again conduct water flow and so there may be a series of deposits each fining upwards. It is also common for the fills of old channels to be organic rich as aquatic and marginal vegetation colonise them.
