Water has several unique physical and chemical properties that have influenced Life as it has evolved. Indeed the very concept of Life on the Earth is dependent on water supply. Although the total volume of fresh water on Earth is only a small fraction of that in the oceans, the biological and environmental role of freshwater systems is considerable. Environmental management of water systems (rivers, lakes, seas) is therefore a necessity to sustain economical development despite the technical challenges. Scientific methodology is based primarily upon the application of basic fluid mechanics to complex systems involving two- and possibly three-phase flows : e.g., sediment transport (watersolid), air-sea interactions (air-water). The writer proves the role of environmental fluid mechanics in gaining a sound understanding of water systems and in solving environmental problems. This is illustrated for a complete catchment system. That is, from upstream to downstream, the problem of reservoir sedimentation, stream re-oxygenation with instream aeration cascades, the impact of tidal bores on estuarine systems and the contribution of breaking waves to airsea mass transfer. In each case, the complexity of the problem is shown and new technological advances are described highlighting the essential role of environmental fluid mechanics. The management of water systems is extremely complex because of a combination of factors including Man's dependence on water, the geometric scale of water systems (up to 1E+7 km2), the range of relevant time scales (from less than 1-s to over 1 E+9-s), and the complexity of the governing equations (non-linearity)
