Tutorial:Water Deposition

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Water deposition
The characteristics of water-laid (alluvial) 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, alluvial sediments tend to be well sorted and individual particles may be rounded and polished by abrasion.

Sheetflow deposits
Because sheet flow can only transport very fine particles, these deposits consist of fine layers of very well sorted fine sands, silts and clays. They are deposited at breaks of slope and may intercalate with alluvial and colluvial deposits.

Vertical accumulation of sheet flow deposits is often slow and bioturbation can destroy the fine banding. Often the only evidence of sheetflow deposition may be a deepening and fining of soil at the base of slopes or upslope of obstructions.

Lake (Lacustrine) deposits
The energy of water flow in a lake is significantly lower than that of the stream or river that feeds it. Because of this the water may no longer have enough energy to carry the sediment it is transporting; hence lakes are areas of sediment deposition.

Where streams enter the lake the energy of the water drops off, so the largest particles are deposited closest to the mouth, whilst finer particles may be carried further into the lake before being deposited, this process creates fans of materials at the mouths of rivers and streams entering the lake that are grade from coarse to finer material.

As well as allocthanous material (external to the lake) transported by streams, lake sediments also include autocthanous sediments (internal to the lake) such as the remains of aquatic animals, plants and plankton, together with chemical precipitates such as calcite.

Sediment deposition in lakes can be highly seasonal reflecting flow conditions in the streams that feed them. This can create annual varves, which can provide a sensitive record of environmental change in the river catchment.

Coastal deposits
Deposition along the coast creates beach, estuarine and deltaic deposits. Deposition and coastal reclamation has meant that some archaeological sites which today are inland were originally in coastal locations.

Beaches can consist of material ranging in size from fine sand to boulders; many beaches also contain high quantities of carbonate material in the form of shells. Beach form is influenced by the size and shape of material, the tidal range and wave processes. The beach gradient can vary between 2o in fine sand, to 20o in pebbles where water infiltrates rapidly rather than washing back out (backwash) as it does on sandy beaches. This also means that pebble beaches are often better sorted and graded than sand beaches. Sandy beaches provide the parent material for beach dunes which are formed by aeolian processes.

In some circumstances, most commonly in tropical environments, the beach particles can be cemented together by calcium carbonate to form beach rock. Care should be taken not to confuse this with eroding conglomerate stone on rocky coasts.

In estuaries low-gradient tidal flats of silt and clay can develop. These flats are formed from clay and silt carried to the coast by rivers. When the rivers reach the sea, the salt in the seawater causes these fine particles to clump together (flocculate), the larger clumps can then settle out in the sheltered waters of the estuary.

Where rivers carrying large amounts of sediment reach the sea, the river water decelerates and its sediment is deposited. This localised deposition forms a large fan of material (delta) that protrudes beyond the coast.

Floodplain deposits
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.

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.

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.

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