IMMEDIATE DOWNSTREAM EFFECTS OF THE PETIT-SAUT DAM ON YOUNG NEOTROPICAL FISH IN A LARGE TRIBUTARY OF THE SINNAMARY RIVER (FRENCH-GUIANA, SOUTH-AMERICA)

The effects of artificially low runoffs in the Sinnamary River, French Guiana, South America, on flow patterns and on richness and abundance of young fish in Venus Creek, one of its main downstream tributaries were examined. After Petit-Saut dam's gates were closed, the areas adj acent to this tributary were never once flooded for the entire duratio n of the rainy season. The daily maximal averages of water speed at th e tributary's mouth were found to be significantly increased. Young fi sh sampled using light-traps were less abundant and less diverse after dam closure. Young Characiformes appeared to be the most affected by these flow disturbances. These findings enabled us to develop a concep tual model of the consequences of impoundment on young fish assemblage s through the modifications of tributaries and associated floodplains hydrology. Because of flow reduction in the river during the first yea r of impoundment, young fish that previously had a tendency of being t rapped in tributaries and flooded areas were then at risk of being flu shed away. The pattern of flow release by dam operations is known to b e very different from natural flow variations. The consequences for do wnstream tributaries will be similar to those of channelization: lack of adjacent flooding areas and higher rates of downstream water transf er. How the recovery of downstream fish assemblages will occur is disc ussed. (C) 1998 John Wiley & Sons, Ltd.