INTERACTIONS BETWEEN FLOODPLAIN FORESTS AND OVERBANK FLOWS - DATA FROM 3 PIEDMONT RIVERS OF SOUTHEASTERN FRANCE

Spatial variations of flow are described and explained within the fore sted margins of three Rhone river tributaries during floods that occur red in 1992, 1993 and 1994. Observations were made on the scale of rip arian forest corridors with cross-sectional analysis, and also on the scale of vegetation units located in the upstream reaches of floodplai n channels using a 4000 m(2) sampling plot. The study of floodplain cr oss-sections does not always confirm relationships between flow depth and altitude or distance from the sampling point to the active channel . Orientation of flow slope Varies too much from one site to another a nd often within each site. Water can flow from the river channel into the forest and vice versa. Floodplain channels characterized by a how depth higher than flow depths observed in neighbouring forests lateral ly supply their margins. On the vegetation patches of the Mellon site (Ain river), food overflows are affected by a line of coarse woody deb ris (CWD) which changes location from pear to year. Thus, position, or ientation and form of floodplain microchannels varied from one year to the next. Floodplain vegetation can be described as patchwork influen ced by overbank flows, which are in turn affected by floodplain topogr aphy, vegetation hydraulic roughness, and by the supply and orientatio n of CWD. An interactive relationship thus exists between discharge an d the forest. This relationship gives rise to a great variety of bioge omorphological processes which may be interpreted as pertinent indicat ors of the lateral connectivity of forested floodplain rivers.