The Itaipu dam consists of a series of different structures with a total length of 7744m. One of these structures is the Left Bank Earthfill Dam (LBED), whose length is 1989m. The LBED is founded directly in the local soil, which consists of a residual soil of basalt. The behaviour of the earthdam is monitored by 9 electric piezometers, 26 standpipe piezometers, 12 water level indicators and 6 flow meters. The flow meters were initially designed to measure the seepage through the dam body. For this purpose, the dam has a system of ditches to collect the water from the internal filters and to lead the water until a flow meter. However, there is water leaving the internal filters just in a little portion of the dam. In addition, there was no knowledge of the origin of the water that is measured in the flow meters, because many alterations were made in the operation of the dam to drain flooded areas. The drained water was conducted to the system of ditches. This work aims to identify the origin of the water measured in the flow meters and understand the seepage through the dam and its foundation. For this purpose a Finite Element Method analysis for eight instrumented sections was performed using the SEEP/W program. In addition, statistical correlations between data from all instrumentation of the LBED and rainfall were made. The main conclusion is that most of the water measured in the flow meters comes from the reservoir through the foundation. And only a little amount of the total seepage through the foundation is measured.