ENSO and the hydrology of the Sinnamary River (French Guiana) during the rainy season: will future El Nino events increase the impact of the Petit Saut dam on downstream fish communities?

This study is based on 27 series of daily water level (DWL) records in the downstream reaches of the Sinnamary River during the November to June rainy season: 22 series prior to dam closure, two series during filling and thre e series during dam operation. Five of these series (4 before and 1 during dam operation) corresponded with El Nino events, six (all prior to dam clos ure) with La Nina events. Before dam closure, monthly DWL were significantl y higher during La Nina events from November to June, and significantly low er during El Nino events in January, February, May and June, than during ye ars with no particular El Nino-Southern Oscillation (ENSO) event. Maximum m onthly DWL were significantly higher during La Nina events than during year s with no particular ENSO event in February, March and May only. The date o f occurrence of the seasonal maximum DWL did not vary significantly with EN SO events. A greater number of days with high DWL were recorded during rain y seasons corresponding with La Nina events than during years with no parti cular ENSO event but El Nino events reduced the occurrence of high DWL in J une only. Dam operation significantly increased monthly DWL in the downstre am reaches of the Sinnamary River from November to January whatever the ENS O event considered. During the 1997-98 rainy season, which corresponded wit h an El Nino event, the dam amplified the impact of lower rainfall by compl etely removing high DWL. It is concluded that in the future, dam operators will have to restore periods of high DWL during rainy seasons characterised by El Nino events, or present nurseries will no longer play their role for sustaining fish diversity in the downstream reaches of the Sinnamary River .