Climate controls the availability of the water in the wetlands and the activity period of the organisms. If the flooding and activity periods match, the floodborne resources can be used by the adapted floodplain biota (e.g., during summer floods). On the other hand, winter floods are generally less deleterious for little-flood-adapted tree species.
In the boreal and temperate regions, freezing and drought in winter and snowmelt floods in spring are predictable drivers of the interplay between surface water and groundwater in riparian wetland hydrology. Ice jams may cause stochastic flood events in winter. Normally, stream runoff is reduced during winter, and groundwater-fed riparian wetlands discharge into the stream channel as long as possible. In wetlands with organic sediments, this water is often loaded with large amounts of dissolved organic carbon. In shallow streams that freeze completely during winter, riparian wetlands may serve as refuges for the aquatic fauna, for example, for amphibians and turtles. Spring snowmelt events generally provoke prolonged flood events that exceed the duration of rain-driven floods. These long floods can connect the riparian wetland water bodies to the stream, so that organic matter and biota become exchanged. At the same time, there is often an infiltration (downwelling) of surface water into the riparian groundwater body.
In seasonal wet-and-dry climates (both Mediterranean and tropical savanna climates) water supply by rainfall is limited to a period of several months during which very strong rainstorms may occur. These events, albeit short, are of great importance for the release of dissolved substances and for the exchange of organic substances and biota between wetland and main water course. Moreover, energy-rich organic matter (e.g., fruits) may become flushed from the terrestrial parts of the catchment into riparian wetlands. On the other hand, flash-floods can cause scouring and erosion of fine sediments (including organic matter). During the dry season, groundwater levels are lower and may cause a seasonal drought in the riparian wetlands. In these periods, the aquatic biota either estivate or migrate into the permanent water bodies, and large parts of the stocked organic matter become mineralized. However, even in strongly seasonal zones, like the Brazilian Cerrado, groundwater supply may be large enough to support permanent deposition of undecomposed organic matter.
The distribution of water-conductive (coarse) and impermeable substrate (bedrock and loam) of the valley bottom influences the thickness of the stagnant water body in the riparian zone and thus the extension of organic matter layers. Permanently humid conditions are found in many riparian wetlands of the boreal zone and in the humid tropics. These permanent riparian wetlands can accumulate large amounts of organic carbon. In tropical Southeast Asia (Malayan Peninsula and parts of Borneo), a special case of riparian wetland occurs, the peat swamps. These swamps develop when mangrove forests proceed seawards, and the hinterland soils lose their salt content. Here, large amounts of organic matter from the trees become deposited and the streams flow within these accumulations (see Peatlands).
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