Channelization and consequences on floodplain system functioning on the Garonne River, SW France

Abstract

This study focuses on changes in flow dynamics and morphology of the river bed in altering floodplain system functioning. Even though direct training works on the Garonne River channel on the upstream reach are moderate, human effects on the river channel have irreversible effects. A few aspects of the complex relationship between the contemporary hydrological and geomorphological river channel changes and the adjacent floodplain system are examined to provide further understanding of the whole fluvial hydrosystem, its habitats and its ecological functioning. The decrease in bedload material resulting from dam construction and industrial gravel extraction after 1960 caused accelerated channel incision in the Garonne River. Despite an increase in channel capacity, bank-full discharge related to a recurrence interval of 1.58 years, as well as low flow discharges, have a tendency to diminish since the beginning of the century. The increase in the channel cross-section reduces overbank flows, especially those generated by high frequency floods. Thus, the regularly flooded areas diminished, causing a decrease in the interaction time between the floodwave and the floodplain. Furthermore, channelization processes have altered floodplain construction processes. Riparian wood die-back during the last 10–15 years is one of the consequences of these hydrological and geomorphological changes of the fluvial ecosystem. © 1998 John Wiley & Sons, Ltd.