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

Abstract

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 three 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 closure) with La Nina events. Before dam closure, monthly DWL were significantly higher during La Nina events from November to June, and significantly lower during El Nino events in January, February, May and June, than during years with no particular El Nino-Southern Oscillation (ENSO) event. Maximum monthly DWL were significantly higher during La Nina events than during years with no particular ENSO event in February, March and May only. The date of occurrence of the seasonal maximum DWL did not vary significantly with ENSO events. A greater number of days with high DWL were recorded during rainy seasons corresponding with La Nina events than during years with no particular ENSO event but El Nino events reduced the occurrence of high DWL in June only. Dam operation significantly increased monthly DWL in the downstream reaches of the Sinnamary River from November to January whatever the ENSO event considered. During the 1997-98 rainy season, which corresponded with an El Nino event, the dam amplified the impact of lower rainfall by completely 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.