CHANNEL METAMORPHOSIS, FLOODPLAIN DISTURBANCE, AND VEGETATION DEVELOPMENT - AIN RIVER, FRANCE

The purpose of this paper is to describe and explain channel metamorph osis of the Ain River in east-central France and the effects of this m etamorphosis on floodplain disturbance and vegetation development. The Ain River is a 195 km long stream originating in the Jura Mountains w hich flows into the Rhone River between Lyon, France, and Geneva, Swit zerland. The lower 40 km of the Ain River, beyond the mountain front, are situated in a valley of outwash deposits where the floodplain is 0 .2 to 1.2 km wide. A complex mosaic of floodplain landscape units has developed. Maps dating back to 1766 and six sets of aerial photographs dated between 1945 and 1991 were used to document changes in channel pattern. Aerial photos and field surveys were used to compile maps of landscape units based on dominant vegetation life-forms, species, and substrate. Six maps dated between 1945 and 1991 were digitized in ARC/ INFO and an overlay was generated to determine the changes in landscap e units as related to channel disturbance. Change from a braided to a single-thread meandering channel probably took place in the period 193 0-1950, The process of river entrenchment has occurred throughout the Holocene but has accelerated in the present century due to shortening of the river course, construction of lateral embankments, and vegetati on encroachment following reservoir construction and cessation of wood -cutting and grazing, The increase in horizontal channel stability cou pled with channel entrenchment have decreased floodplain disturbance a nd lowered the water table by approximately one meter. Pioneer and dis turbance-dependent landscape units have experienced a more terrestrial -like succession to an alluvial forest. Abandoned channels have also b een replaced by alluvial forests. On poorly drained soils, shrub-swamp communities of willow and hydrophytic herbaceous plants have been rep laced by mixed forests of ash, alder, black poplar, and oak. On well d rained alluvial soils, ash and oak dominated hardwood forests have dec lined in favor of mesophytic stands of black poplar. All types of vege tation, but particularly dry grasslands-shrublands, have been cleared for mines, campgrounds, agriculture, and other types of development. U sing several measures, landscape diversity decreased between 1945 and 1991.