PALEOFLOOD HYDROLOGY AND EXTENDED DISCHARGE RECORDS

There have been remarkable recent advances in studies of the effects of ancient floods on natural recording systems, allowing the extension of discharge records over the past 10,000 years. Although the term "palaeoflood hydrology" was not even introduced until 1982, various methods of palaeoflood analysis now have worldwide application. Extensive and detailed palaeoflood hydrological investigations have been performed in the United States, China, Australia, India, Spain, Israel, Japan and South Africa. Paleoflood hydrology is the study of past or ancient flow events that occurred before direct measurement by modern hydrological techniques. This subject has experienced a revolution in its development in recent years. There are now numerous techniques of very high accuracy for the estimation of past flood discharges that effectively extend stream-gage record of floods to time scales of several thousand years. Reconnaissance work shows the potential for much more application in South America, southern and central Europe, northern Africa and southwestern Asia. Although long historical records and palaeoflood data do have the necessary record lengths to test the hypothesis of increasing extreme flooding, these aspects of science have received minimal attention and minimal funding, thereby contributing to the current nonscientific, nontestable paradigm of global extreme flood assessment. A very preliminary survey of historical and palaeoflood records suggests the following (though the database is woefully inadequate to defend these conclusions in a rigorous manner): (1) very large floods (those exceeding some threshold value) seem to cluster on time scales of decades and centuries, (2) there are some regions, particularly in arid regions and the tropics, where the most recent century shows a cluster of extreme flood magnitudes, and (3) the floods of recent years do not generally exceed the magnitudes of those in the current cluster, or those of past clusters, and much larger floods are usually indicated in the past. These conclusions are sufficiently controversial and important enough to warrant the expenditure of more resources on the collection of relevant historical and palaeoflood data.