Globally, floodplains may be of greater value to society than any other ecosystem type. This is because of the critical role that interactions between floodplains and associated streams play in maintaining supplies of clean water. While that role is conceptually simple, the processes which define interactions (i.e., floodplain functions) in aquatic-terrestrial ecotones are exceedingly complex. Consequently, it is necessary to develop some understanding of the ecological mechanisms behind those interactions in order to fully appreciate the importance of floodplain ecosystems. To that end, the goal of this article is to provide a first-iteration overview of flood-plain form and function (Figure 1).

A key concept is that floodplains and associated streams are both causes and reflections of the other's characteristics and functioning. As an example, the climate and geomorphology of a landscape will define the hydrology and initial chemistry of streams. Stream characteristics determine the hydrology, soil characteristics, flora and fauna, and biogeochemistry of the floodplain. In turn, biogeochemical feedback from floodplains to streams helps define the environment seen by aquatic flora and fauna. Thus, a strong interdependency exists between aquatic and terrestrial components of riparian ecotones.

It is critical to understand that land clearing and development, construction of dams and impoundments, pollutant export, and other human activities constitute major influences on streams and floodplains. In some cases, these will override the original hydrology, biogeo-chemistry, and ecology. As an example, the original hydrology of a riparian system could be dramatically altered by the construction of bermed roadways that cross streams without adequate provision for through flow. Since hydrology is the primary driver of all flood-plain functions, corresponding changes in net primary productivity (NPP), species composition of animal and plant communities, and biogeochemistry could be expected to follow.

Figure 1 Panoramic view of the Timpisque River and the Palo Verde Marsh in Costa Rica.
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