Açude Coremas, officially known as Açude Estevam Marinho, is a large reservoir located in the municipality of Coremas, in the microrregião de Piancó in the state of Paraíba, Brazil. Together with the adjacent Açude Mãe d'Água, it forms a single continuous water surface connected by a 237-meter spillway channel capable of transferring up to twelve cubic meters of water per second. The combined system holds a maximum storage capacity of approximately 1.358 billion cubic meters and drains a watershed of 8,700.34 km², making it the largest water reserve in Paraíba and the fifth largest in Brazil. The Coremas reservoir alone has a capacity of around 720 million cubic meters, fed by the rio Piancó and the broader rio Piranhas-Açu basin, while Mãe d'Água is formed by the barramento of the rio Aguiar.

The dam structure itself is of the zoned earthfill type with a central reinforced concrete core, reaching a maximum height of 47 meters and extending 1,550 meters in length. Construction was carried out by DNOCS between April 1937 and May 1942 under the direction of engineer Estevam Marinho, and upon completion it was considered the largest dam in Brazil, a distinction it held until 1960.

The reservoir is used for recreational boat trips, with local operators offering panoramic excursions on the water from Coremas. The vast expanse of open water surrounded by the semi-arid sertão landscape gives the place its character as a "mar no sertão" — an inland sea in the backlands. The area is notable for its sunsets, with the low light reflecting across the water and the dam's concrete structure creating striking visual conditions. The reservoir supports fishing activity as well. Average annual rainfall in the region is approximately 860 mm, and the annual mean runoff is around 632 million cubic meters, with a runoff coefficient of just 9.2%, reflecting the semi-arid climate. Paddlers should be aware that water levels can vary considerably depending on seasonal rainfall patterns in this semi-arid zone.