The Rio Grande de Loíza (English: Great River of Loíza) is a river in the island of Puerto Rico. It is the largest river in Puerto Rico by volume. It is situated on the north coast of the island. It flows from south to north and drains into the Atlantic Ocean, a few miles east of San Juan.Rio Grande de Loíza runs for approximately 40 miles (64 kilometers). It has its origin in the municipality of San Lorenzo at an altitude of approximately 3,500 feet (1,073 meters) above sea level. It runs through the municipalities of San Lorenzo, Caguas, Gurabo, Trujillo Alto, Carolina, Canóvanas, and Loíza forming the Loíza Lake along its route, making it the second longest river on the island, behind Río de la Plata.
In the 1898 Military Notes on Puerto Rico by the U.S. it is written that the "limits of the Loisa River are: On the east, the sierra of Luquillo (situated near the northeast corner of the island); on the south, the sierra of Cayey, and on the west, ramifications of the latter. It rises in the northern slopes of the sierra of Cayey, and, running in a northwest direction for the first half of its course and turning to northeast in the second half, it arrives at Loisa, a port on the northern coast, where it discharges its waters into the Atlantic. During the first part of its course it is known by the name of Cayagua."The river was commemorated in a poem by Julia de Burgos.
Flood control project
In mid 2018, the United States Army Corps of Engineers announced it would be undertaking a major flood control project of the river, with a budget of $250 million.