Floodplain biogeomorphology

Abstract

Floodplains are unique ecosystems because of their linear form, the sometimes extreme dynamism of their geomorphology and because they process large fluxes of energy and materials from upstream areas. This article focuses on the importance of hydrological inputs to floodplains through 1) their influence on the arrangement of landforms and vegetation communities and 2) the connections between flooding regimes and the regeneration and turnover time of floodplain vegetation. Many researchers have demonstrated close links between the arrangement of vegeta tion communities and sedimentary landform types, elevation, soil characteristics, tolerance to flooding and availability of soil moisture. It is suggested that plants on floodplains are found along a combined gradient of available moisture and oxygen which can be viewed simultaneously as a flooding frequency gradient and a complex soil moisture gradient. Discussion of experi mental work on floodplains demonstrates the importance of these gradients to a range of flood plain species in different environments. The relationships between these environmental gradients and the apparent high level of overlap between planform patterns of landforms and vegetation communities on floodplains are related to lag times in different parts of vegetation communities. Flood regimes greatly influence the availability of areas suitable for vegetation regeneration from year to year and the age structure of floodplain communities over decadal time frames. Biotic factors also influence biogeomorphological relationships on floodplains and range from sediment- trapping by vegetation to the impacts of beaver and grazing animals on floodplain hydrology and vegetation. Restoration of floodplains is high on the agenda in many countries and it is argued that, for sustainable results, restoration of hydrological pathways is essential. Planned flood releases below dams in several African countries have had varied success rates but the develop ment of models for managing flows to achieve different restoration targets is the start of an integrated approach to restoring complex floodplain ecosystems.