Estimating the Water Requirements for Plants of Floodplain Wetlands

Section 1:IntroducingFloodplainWetlandsUnderstanding floodplain geomorphology and hydrology is thekey to understanding ecological diversity of floodplain wetlands,and for understanding the vegetation. In this section, the keyfeatures of geomorphology and hydrology are introduced, toshow the diversity within and between floodplains. Floodplainwater balance, which is an important topic of this guide, is alsointroduced, and its link to water regime outlined.What are floodplains?Floodplains are fluvial depositional environments formed over longperiods from sediments transported by rivers in flood. In terms of riverflow velocity, floodplains are low energy environments, a result of theirvery low slopes and low relief (Note 1).The rate of floodplain formation depends on the prevailing flow regimein the river and on the nature of sediment delivery from the uppercatchment. Most floodplains are polyphasic ie. they have been formedunder variable flow, climate and deposition conditions, whereas amonophasic floodplain is one formed under just one set of conditions.Old floodplains are unlikely to be monophasic, but young ones mightbe. The age of floodplains varies. The Willandra Lakes on the LachlanRiver distributary are at least 55,000 years old, whereas coastalfloodplains of the Northern Territory are 3–6,000 years old. Anappreciation of the age of a floodplain and its forms is useful forunderstanding its ecological diversity. Floodplains on large rivers arelarge, covering 100,000–200,000 hectares.Fluvial forms on floodplain wetlands are mainly relict channelfeatures, such as billabongs and anabranches (eg. Figure 1). Billabongs,which include ox-bows, cut-off meanders and small lentic features, canbe numerous and quite diverse. If billabongs originate from a period ofhigher discharge, their dimensions will exceed those of thecontemporary river, and they will have different sedimentcharacteristics. Floodplain age and formation can be a short guide tobillabong diversity. Anabranches are channels that leave then re-join thepresent river. Typically, these flow at times of high discharge. Often, butnot always, an anabranch is a paleochannel and its dimensions aredifferent from the present river. Anabranches are a notable feature of themajor rivers in the Murray–Darling system.Note 1Floodplain classificationLack of scientific and ecologicalstudies of floodplains have hinderedunderstanding of floodplain ecology.An example of this is that as yet thereis no Australia-wide system offloodplain classification.In general, the most effectiveclassification systems are those thatare based on their formativeprocesses, also known as functionalor genetic classifications. Forfloodplains, this means aclassification system based on streampower, discharge and sedimentcharacteristics.A genetic floodplain classificationsystem has been developed byNanson and Croke (1992). TheirClass C, low-energy cohesivefloodplains, covers terminal wetlandsand most of the floodplains in theMurray–Darling Basin.Section 1: Introducing Floodplain Wetlands 11