Introduction
Oceans dominate the water landscape of Earth, but scattered across the terrestrial landscape are less obvious water ecosystems of many kinds that are nevertheless vital to all of humanity. Of the 1.4 billion cubic kilometres of water on Earth, 97% is seawater with only limited potential for terrestrial use (Pearce, 2006). Two-thirds of the remainder is locked in ice caps and glaciers, and one-third is in liquid form – most stored deep below the earth's surface in aquifers. The minute remaining liquid fraction, not much more than 200 000 cubic kilometres, is stored in lakes (90 000 cubic kilometres), soils and permafrost (90 000), the atmosphere (13 000), a range of wetland types (11 000), rivers (2000), and living organisms (1000) (Pearce, 2006). Occupying less than 1% of the Earth's surface (Reaka-Kudla, 1997; Dudgeon et al., 2006), rivers, lakes, floodplains, swamps, lagoons, pans, bogs, seeps, estuaries and other kinds of surface inland waters enrich our lives with their variety and beauty, but also, along with groundwater aquifers, provide the freshwater we need for our lives, homes and industries.
Of all aquatic ecosystems, rivers are the most prominent providers of water because of the continual flowing supply and they are, in all but the most arid regions of the planet, among the most pervasive features of the physical landscape. They are giant conveyor belts (Kondolf, 1997), moving up to 40 000 cubic kilometres of water from land to sea every year and carrying associated sediments and chemical loads.