Rivers form a hydrological mosaic, with an estimated 263 international river basins covering 45.3% (~231 059 898 km²) of the land surface area of the Earth, excluding Antarctica (UNEP, Oregon State University et al., in preparation). The total volume of water in the world's rivers is estimated at 2 115 km³ (Groombridge and Jenkins, 1998).

Reservoirs are artificial lakes, produced by constructing physical barriers across flowing rivers, which allow the water to pool and be used for various purposes. The volume of water stored in reservoirs worldwide is estimated at 4 286 km³ (Groombridge and Jenkins, 1998)

Wetlands include swamps, bogs, marshes, mires, lagoons and floodplains. The 10 largest wetlands in the world by area are: West Siberian Lowlands (780 000-1 000 000 km²), Amazon River (800 000 km²), Hudson Bay Lowlands (200 000-320 000 km²), Pantanal (140 000-200 000 km²), Upper Nile River (50 000-90 000 km²), Chari-Logone River (90 000 km²), Hudson Bay Lowlands in the South Pacific (69 000 km²), Congo River (40 000-80 000 km²), Upper Mackenzie River (60 000 km²), and North America prairie potholes (40 000 km²) (Pidwiny, 1999).

The total global area of wetlands is estimated at ~2 900 000 km² (Groombridge and Jenkins, 1998). Most wetlands range in depth from 0-2 metres. Estimating the average depth of permanent wetlands at about one metre, the global volume of wetlands could range between 2 300 km³ and 2 900 km³.