Abiotic aspects of channels and floodplains in riparian ecology

1. The ecology of riparian zones is enormously influenced by the heterogeneous sedimentary structures and associated complex hydrologic flow paths that mediate surface- and groundwater exchanges. Sedimentary structures form a three-dimensional, dynamic framework that controls subsurface flow and the vertical and horizontal exchange of water between channels and floodplains in gravel bed rivers. The modern structure of the bed sediments reflects the legacy of cut and fill alluviation for a particular river basin. 2. Highly permeable sedimentary textures, particularly open framework gravels, allow rapid exchange between surface and groundwaters. 3. Ground penetrating radar provides high resolution information on the nature and three-dimensional distribution of the sediments within the shallow subsurface (4–25 m) of gravel bed rivers. Bed sediments can be mapped at the decimeter scale. 4. Exchange and mixing of ground and channel water occurs along losing, gaining and flow-through reaches as determined by the hydraulic gradient and transmissivity of the bed sediments. 5. Spatial and temporal patterns of surface- and groundwater interactions can be quantified by mass flux measurements and by assessing geochemical contrasts. Natural tracers, such as temperature or radon, are well suited for mapping exchange sites and quantifying interactions. Artificial signals produced by injecting anions, like chloride, bromide and organic dyes are also useful. 6. The study of riparian ecosystems requires an understanding of the geomorphic structures and processes that build and maintain bed sediments and flow pathways through them.