TABLE OF CONTENTS The quantitative understanding of mixing and dispersion processes in rivers is a key element in successful strategies for water quality control and management. Despite great advances that have been made over the last half century there remain a number of challenging research questions that need to be resolved.
For continuous discharges, that are usually subject to a routine permitting process by water authorities, the specification of transverse mixing is most important for meeting given mixing zone regulations. Secondary current and coherent eddy structures, especially in curved or meandering channels, e.g. in the wake of structures or islands, will influence mixing processes.
Many more open research questions concern the longitudinal dispersion process due to instantaneous or accidental pollutant releases in river systems with complex morphology. In order to decrease the present parameter uncertainty systematic studies are needed on the effect of river heterogeneities, such as meandering, overbank floodways and side arms, partly vegetated flow zones, and groin fields, all with variable hydrological conditions, such as water levels.
Finally, the interaction of these physical processes with the chemical and biological transformations that govern the fate of particular chemicals need to be addressed. This includes pollutant sorption processes with fine organic or inorganic suspended or bottom sediments and with biota.
