• Parnaíba river: after the San Francisco River basin, the Hydrographic Region is hydrologically Parnaíba the second most important of the Northeast. It covers the state of Piaui and the states of Maranhao and Ceara. The region covers an area of 344.112km ², equivalent to 3.9% of the country, and drains almost all of the state of Piauí (99%) and part of Maranhão (19%) and Ceará (10%). The Parnaíba has about 1,400 km long and most of its tributaries are located downstream from Teresina are perennial and supplied by rainwater and groundwater. The main tributaries of Parnaíba river, are Poti, Portinho, Canindé, Piauí, Longa, Balsas, and Uruçu-Preto and Gurguéia. The three last mentioned rivers are included in the Project area;
