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.

The influence of hydrological regimes on sex ratios and spatial segregation of the sexes in two dioecious riparian shrub species in northern Sweden

“…It is well established that on many floodplains, regeneration is periodic and related to provision of very particular conditions of water availability and disturbance patterns during the first year of growth following germination. Thus, willows require open, well-watered but not waterlogged sites for successful regeneration; the seedlings must then not be subjected to physical disturbance by flooding or ice until they are well established (Johnson et al 1976;Bradley and Smith 1986;Mahoney and Rood 1991;Hughes 1997). …”

Mechanisms of vegetation uprooting by flow in alluvial non-cohesive sediment

“…Besides providing natural habitat for various river ecosystem species (Hughes, 1997), riparian trees interact with river streamflow thus contributing to a number of fundamental feedback dynamics. Examples are the lateral and longitudinal connectivity between the river and the floodplain, the control of water quality by biological filtering, the influence on sedimentary processes (Edwards et al, 1999;Schnauder and Moggridge, 2009) and the spatial spreading of species (Johnson, 2000;Mahoney and Rood, 1998).…”

“…On the contrary, erosion of vegetated non-cohesive alluvial sediment such that forming river bars and islands has only recently been attracting the interest of scientific communities with regard to river restoration projects (e.g. Palmer et al, 2007) and the related implications for the connected riparian ecosystem (Decamps et al, 1988;Hughes, 1997;Tockner and Stanford, 2002).…”

Effects of hydrologic modifications to riparian plant communities in a large river system in northern China

“…Water flow is a major structural factor in ecological communities and is an important driving force of com-munity composition in riparian habitats, because of the temporal and spatial variability of flood and drought (Walker et al 1995;Hughes 1997;Poff et al 1997;Richter et al 1997;Ward 1998;Nilsson and Svedmark 2002). A number of eco-hydrological studies have identified relationships between riparian vegetation and hydrologic parameters, including frequency, timing, duration, rate of change, and magnitude of base flows (Nilsson and Svedmark 2002;Elmore et al 2003).…”