Aspects of the biogeochemistry of estuaries from a variety of environments, from the tropics to the Arctic, are discussed. In most cases the courses of these rivers have been altered by dams or diversions; the results of these changes on the nature of the estuary are also discussed, where such data is available. In the case of the Tasmanian rivers, the estuary of the Huon, a largely untouched river, is contrasted with that of the Derwent, a river heavily influenced by industry. The future state of all of these estuaries may be a sensitive indicator of shifts in global weather patterns.