Valley Size Morphology and Connectivity

The common textbook pattern of steep valleys in the upper sections of the streams and open, shallow floodplains in the lower river sections holds true only for very few cases in nature. Rather, we find these two valley types interspersed in an alternating pattern like 'beads on a string. Shallow areas are more likely to bear extended riparian wetlands; however, if groundwater levels are high enough, even steep valleys may be covered with wetlands. The morphology of riparian wetlands can be described by the entrenchment ratio (i.e., the ratio of valley width at 50-years flood level to stream width at bankfull level) or by the belt width ratio, that is the distance between opposing meander bends over a stream section to stream width at bankfull level. Fifty-years flood often intersect the terrace slope.

Riparian wetlands of different catchments may be linked with each other through swamp areas (e.g., in old eroded landscapes of the Brazilian and Guyana Shields in South America) so that biogeographical barriers can be overcome by aquatic biota even without a permanent connection between the water courses. The term connectivity describes the degree by which a floodplain water body is linked to the main channel. Riparian wetlands may also be connected to the stream, either in a direct connection by a short channel, or indirectly by a longer channel which may be intercepted by a pond. In some cases, these channels can be cryptic/hidden when they are formed by macropores in the organic soils. Alluvial riparian wetlands may be connected to the stream via the hyporheic interstitial zone provided that the sediments are coarse enough to conduct water. Wetlands without any ofthese pathways exchange water, biota, and organic matter with the main channel during overbank flow ofthe stream. Purely aquatic organisms depend on the existence of connection channels to migrate between wetland and main water body. For example, amphibia are especially sensitive to fish predation, so that the highest biodiversity ofamphibia is found at riparian wetland habitats with the lowest accessibility for fish.

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