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O ne of the quirks of the Earth’s stratigraphic recording system is that it operates in fits and starts, archiving surface conditions for a while and then suddenly switching off. The sedimentary ‘record’ button is evidently pretty finicky. It is sensitive to external controls, but also to the inter- nal dynamics of the sediment-deposi- tion system. An intriguing example of how the recording process works is given on page 493 of this issue. Aalto et al. 1 show how changes in the way that rainwater is delivered to a river system, driven by the El Niño/Southern Oscilla- tion (ENSO) climate cycle in the Pacific Ocean, can switch one of the major processes of sedimentation on and off. River flooding seems a fairly straightforward process — so much so that one could easily overlook the fact that no obvious natural law decrees how often and by what means a river should flood, or, for that matter, that it should flood at all. An alluvial river channel (one formed from its own sediment) and its adjoining flood plain are self-formed from the interplay of water, sediment and vegeta- tion. The dynamics of flooding results from the way in which the channel–flood-plain system organizes itself. In many alluvial rivers, sedimentation associated with flood- ing creates ‘natural levees’, low ridges that run along the edges of the channel. These levees are naturally formed counterparts of the familiar artificial levees built to control flooding in populated areas. They raise the ‘overtopping’threshold ofthe channel and in so doing create a pressure-head that helps drive water onto the flood plain once the levee is breached. So, rather than gradually
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This note was uploaded on 07/23/2008 for the course GEOSC 203 taught by Professor Anandakrishnan during the Fall '07 term at Pennsylvania State University, University Park.

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