The River Madeira is formed by a wide network of smaller rivers which originate in Brazil, Bolivia and Peru. The River Mamoré (the largest) and River Beni (the most abundant in terms of water) are worth mentioning along with the Guaporé, Madre de Dios and Orthon. All originate at high altitudes and their courses run along many declines and waterfalls. Some of the springs from which these rivers originate are located in the slopes of the Andes (they drain 35% of the thawing ice from the mountain range). When these rivers reach the plains they meander. The Santo Antônio hydroelectric plant is located in this stretch. Downstream from the HEP, the Madeira runs across flatter land until it flows into the Amazon.
The waters of the River Madeira are clean but murky as they carry an enormous quantity of sediments from the Andes through the River Beni. The name of the river ("wood" in Portuguese) comes from the large number of branches of trees, leaves and large trunks which it transports.
The river is at its lowest level in September. The highest level is in March and the average seen during the year is 23,000 m³/s of water. The level of the Madeira is influenced by the local rainfall system and the thawing of the slopes of the Andes.
The region where the Santo Antônio plant is located has average temperatures which are nearly always high, close to 25 C. However, the peaks of summer and winter can bring considerable variations, with a maximum temperature of 35 C between September and November and a minimum of 9 C between June and July.
Rain is relatively scarce in winter in contrast to the period between December and January when there is heavy rain. The smooth relief of the region is more fragmented on the left bank of the River Madeira, with areas rising to 100m to 150m high. The air is always humid, varying between 81% in July and 89% in December.
There are generally clouds at any time of the year as the Amazon Basin produces the largest coefficient of nebulosity in the whole of Brazil. There are greater chances of sunny days in winter.