The chemical composition of natural waters is determined by several sources. Many terrestrial aquatic organisms disturb the substratum (soil, sediment) in which they grow and live, extracting chemical substances from the environment and excreting others that may affect the chemistry of natural waters. Also human influence could influence this precarious equilibrium and cause important effects on the communities using the water resources. This work was the first to study systematically the water characteristics of Lake Ipê, where the water regimen is regulated by Itaipu (southwest-downstreams) and Ilha Solteira (northeast-upstreams) dams. The main tributary of the Parana River is the Tiete River coming from S. Paulo city, about 300 km upstreams. S. Paulo is the largest city of South America with over 10000000 inhabitants and its largest industrial park, with practically no treatment of domestic and industrial wastewaters. Several studies have been performed in the Tiete River basin, always upstream from the Ilha Solteira dam. However knowledge is lacking of the lake waters downstream from the dam. The river-floodplain systems of tropical regions are submitted to very particular climate and environmental conditions, causing a remarkable temporal variation in the physical-chemical and biotic parameters. Alterations in these parameters are related mainly to changes in the hydrometrical levels, called “flood pulse”, 1 having two phases, flood and drought. 2, 3 The seasonal behavior in a tropical flood plain region is controlled by factors different from those that act in temperate regions, where the determining parameters are temperature and photo-period. 4 The objective of this study was to explain the seasonal variation of abiotic parameters and of the most important metals (Fe, Mn, and Cd) during a complete hydrological cycle of Lake Ipê (13 months) supported by Principal Components Analysis (PCA) and MINTEQA2 analysis.