MÉTODO SOFTWARE GEO5

The geomorphology of riverine wetlands plays an important role in their maintenance and functioning. Unlike buffer zones in more terrestrial settings, the fluvial forces of erosion and sediment reworking alter the structure of natural riverine wetlands over geologically rapid (hours to centuries) timescales.

River channels

River channels are of three main types: braided, meandering and anastomosing (Salo, 1990). Braided

channels are characterised by a network of constantly shifting, low sinuosity water courses and are

predominantly found in rivers of arid and semi-arid climates, along mountain forelands and along the outwash plains of ice caps and glaciers. Although small wetlands can form in inactive portions of braided channels, the riparian zones adjacent to braided channels are generally not wetlands

(NRC 1995). Meandering channels occur along low gradient rivers in humid environments and are

characterised by a helicoidal flow, a coiling type of water movement that results in erosion of the concave outer bank and deposition along the convex inner bank. Meandering is the result of a river’s adjustment to its environment in order to carry its load most efficiently and tends to follow certain mathematical rules with regard to meander wavelength, meander amplitude, stream flow volume, stream flow velocity, channel width, channel depth, floodplain slope and river gradient. Meandering rivers have a single primary channel, high suspended load to bedload ratio, cohesive bank material

and relatively steady discharge (Reineck and Singh, 1980).Anastomosing channels are characterised by

multiple channels that separate and reconnect and occur in large rivers like the Amazon which show a lack of channel competition along their middle and lower reaches (Salo, 1990). This channel form results from a strong flood regime and a dominance of suspended sediments over bedload sediments (Reineck and Singh, 1980).

River floodplains

Floodplains are sedimentary environments, of net deposition, associated with river flooding, created

by the processes of stream meandering and overbank flooding (Salo, 1990, Costaet al., 1995). As water

flows around a river bend, the current velocity increases on the outer edge of the curve, leading to

erosion, and decreases on the inner edge, leading to the deposition of a point bar. The lateral and

downslope migration of meanders results in the development of meander scrolls, in which point bar

ridges alternate with low-lying sloughs or swales (Morisawa, 1985). When the migrating river channel cuts the meander from a different angle, a new meander loop may start to form near the old one. As the radius of a new meander loop becomes larger, the old meander may become cut off and

abandoned, forming anoxbow or cutoff lake.

When streams and rivers overflow their banks, sediment is deposited adjacent to the stream channel as a natural levée. Sediment size decreases as current velocity slows with increasing distance from the

channel (Salo, 1990). Levées separate the channel from low-lyingbackwater areas.

Historical maps and aerial photos can be used to document natural channel migration over time

(Braga and Gervasoni, 1983, Johnstonet al., 1992). This can provide insights into past events that have

influenced the present characteristics of riverine wetland soils and vegetation. For example, a map of wetland soils along the East Branch Stream draining into White Clay Lake in northern Wisconsin, USA showed that the location of alluvial soils (Fig. 1a) was anomalous with stream location. However, inspection of historical aerial photos revealed that the contemporary stream outlet had recently migrated to a new location 40 m south of its position for the previous 30 yrs (Fig. 1b), indicating that soil characteristics associated with levée deposition merely lagged behind stream relocation. Historical maps and aerial photos are more commonly used to document artificial channel alteration, such as the channelisation of rivers to prevent meandering (Shäffer, 1973; Sedell and Frogatt, 1984; Fortuné, 1988;

Pinayet al., 1990).

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C.A. Johnston, J.P. Schubauer-Berigan and S.D. Bridgham

Figure 1a. Wetlands bordering the East Branch Stream (second-order) in Cecil, Wisconsin, USA: Soil types described by Johnston et al. (1994a). Fluvaquents = alluvial soils,

Borosaprists = organic soils, Haplaquents = wet mineral soils. The dashed line shows the contemporary stream channel and the shaded areas are post-glacial beach ridges.

Figure 1b. Wetlands bordering the East Branch Stream (second-order) in Cecil, Wisconsin, USA: Historical stream channel location, determined from aerial photography (after Johnston

et al., 1984b).

Haplaquents

Haplaquents

Haplaquents

Haplaquents

White Clay Lake

Borosaprists Borosaprists Borosaprists Borosaprists Fluvaquents Fluvaquents Fluva quents UPLAND N 0 100 ft 0 40 m Stream Channel UPLAND 0 100 ft 0 40 m N Beach Ridge Beach Ridge

White Clay Lake

Stream Channel Present Stream Channel 1966 Stream Channel 1966, 1952, 1938 Stream Channel 1938 Stream Channel 1952 Stream Channel Present

Some of the largest riverine wetlands occur where rivers flow through flatlands formed by other geomorphic processes. Fluvial forces play a smaller role in the genesis of the river floodplain, but river flooding is the major water source to the wetland. Examples include The Llanos in Columbia, where the Orinoco River flows through a sedimentary basin and the Red-Saskatchewan-Nelson River system in Canada, which flows across former glacial lake beds and the Hudson Bay lowlands. In the eastern US, extensive bottomland hardwood forests and cypress swamps line the rivers that drain the Piedmont and flow across the Coastal Plain to the Atlantic Ocean: the Roanoke, Chowan, Little Pee Dee, Great Pee Dee, Lynches, Black, Santee, Congaree, Altamaha, Cooper, Edisto, Combahee, Coosawhatchie and the Savannah (Mitsch and Gosselink, 1993).

Deltas

Deltas are formed where rivers flow into lakes or oceans, depositing their sediment load as a

consequence of decreasing water velocity. Fan-shaped arcuate deltas, such as the Nile Delta, have

multiple, shifting distributary channels. Branchingbirdfoot deltas, such as the Mississippi delta, form

where rivers carrying a large load of suspended sediment flow within a relatively stable distributory, confined by natural levées.

Deltas are associated with some of the world’s largest wetlands (Table 1). Deltas contain a combination of salt water, brackish and freshwater wetlands, depending on the relative inputs of ocean and river water. Due to their strategic location at the coastal intersection of major rivers, most deltas are the sites of major cities and have been extensively altered by man. Conversion of the Rhine River Delta to cultivation and urbanisation in The Netherlands, for example, is legendary.