There is a growing body of knowledge on the distribution and nature of vegetation along river banks and across floodplains. The vegetation changes with distance from the river’s edge in a series of lateral zones on floodplains and river banks. The main drivers of zonation are usually seen as two-fold. Arguably, the primary driver is river flow, with the magnitude and timing of flow, the area of land it inundates, and the velocity, depth and duration of inundation all influencing what species can live where. The geomorphological nature of the river channel and surrounding land is also important, as is the nature of the soils, dictating where water can reach and for how long. Through the interplay of flows and landscape, river banks are inundated and exposed at different times of the year, providing a range of conditions that are exploited by different flora. Floodplains exhibit similar vegetation patterns, with their outer rims supporting quite different species to the middle and inner sections. The main distinction between the zones seems to be between two major ones: a lower (or inner, on floodplains) zone that is inundated most years and a higher (or outer) zone that is inundated less frequently.
Some authors have divided these major zones into sub-zones. For example, two communities within the lower zone that were inundated ca. 40 % and 5-25% of the year, and two within the higher zone, one of which was flooded every 2-3 years and the other less frequently. Several authors noted that the border between the two major zones appeared to be at the elevation reached by the channel-forming or bank-full flood, which has a return period of about 1 in 2 years in strongly perennial rivers, or longer in rivers with more flashy hydrographs.
The botanical nature of the two main zones along rivers differ as the nature of the vegetation community in each zone reflects the life-history traits (recruitment, persistence, reproduction) of individual species, each with its different response to flow and/or inundation. The lower zone tends to comprise graminoids (sedges, rushes and grasses) while the higher zone comprises woody shrub and tree species many of which occur only or mainly along rivers. Thus, zonation reflects the occurrence of groups of species that respond in the same way to prevailing allogenic (hydrogeomorphological) factors, with autogenic (plant induced) factors overlying this.
Links between vegetation zones along rivers (and across floodplains) and the flow regime of a river are used to predict vegetation changes that are likely to occur with flow modifications such as those caused by dams. Such predictions form part of Environmental Flow Assessments and are vitally important when water resource decisions are made, especially in developing countries where millions of people use riparian natural resources for food, construction materials, shelter, crafts and firewood. Assessing development-driven changes to river flow and the knock-on effects on the riparian zone is becoming an important component of environmental impact assessments, and the greater the geographical coverage of the generalisations that can be made the greater the likelihood that they can and will be validly used in decisions on water-resource developments.