Fundamentals of Fluvial Geomorphology by Ro Charlton & Ro Charlton

Author:Ro Charlton & Ro Charlton [Charlton, Ro]
Language: eng
Format: mobi
Tags: Agriculture &#038, Environmental Sciences
Publisher: Taylor and Francis
Published: 2007-11-11T16:00:00+00:00

Depositional environments

Although deposition does occur in the production and transfer zones of the fluvial system (see Figure 2.1) it dominates in the deposition zone, where there is a decline in gradient and energy availability. Large-scale deposition leads to the development of characteristic landforms, including floodplains, alluvial fans and deltas. Within channels, bars represent smaller-scale depositional features. They are commonly found on the inside of meander bends, along the edges of channels, and where tributaries join the main channel. Braided channels are characterised by numerous midchannel bars.

Floodplains border the channels of alluvial rivers and are formed from a mixture of in-channel and overbank deposits. Their development and evolution, which will be examined in Chapter 8, is governed by a number of factors, including the supply of sediment (volume and calibre), the energy environment of the channel, and the valley setting. Sediment is laid down by rivers as they migrate across the floodplain, being deposited on the inside of meander bends or when braid bars are abandoned. These channel deposits are relatively coarse in comparison with the much finer sediment that is laid down by overbank flows. Processes of erosion can also be significant in reworking sediment or in removing part, or all, of the floodplain surface.

Alluvial fans are typically found in situations where an upland drainage basin flows out onto a wide plain (Plate 7.5). The sudden change from confined to unconfined conditions leads to flow divergence, while mean flow velocity is decreased by the reduction in slope. The resultant deposition leads to the formation of a conical feature with a convex cross-profile. Most fans have a radius of less than 8 km, but can be more than 100 km wide in some cases. Where a number of individual fans develop along a mountain front, they may grow laterally and coalesce to form a sloping apron of sediment called a bajada. Fans are commonly found in dry mountain regions, where an abundant sediment supply is associated with extreme discharges and frequent mass movements. Frequent shifts are often seen in the position of the braided channels that cross the fan surface, although only part of the fan surface may be active during a major flood event. In long profile, the slope is steepest at the fan head, progressively decreasing along the length of the fan. There is also a down-slope reduction in sediment size, although deposits are coarse and poorly sorted. Incision and fan head trenching is associated with decreases in sediment supply, or increases in slope. Such changes can be caused by tectonics, climatic variations, a fall in regional or local base level, or human activity. In the absence of external change, the progressive lowering of the landscape will also result in a decline in sediment yield over time.

Arid fans are generally smaller and steeper than those found in humid regions, a large-scale humid example being the Kosi Fan on the southern Himalayan mountain front. This covers an area of 15,000 km2 and formed where the Kosi River descends onto the wide alluvial plain of the Indus.

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