High Quality Content by WIKIPEDIA articles! The Río Grande de Santiago is the longest river in Mexico, measuring up 433 km (269 mi) long. The river begins at Lake Chapala and continues roughly north-west through the Sierra Madre Occidental, receiving the Verde, Juchipila, Bolaños, and other tributaries. The Aguamilpa dam was built in 1997 creating a reservoir covering a large part of the territory of the municipality of El Nayar in Nayarit. It descends to the coastal lowlands, passing by Santiago Ixcuintla and empties into the Pacific Ocean, 16 km (10 mi) northwest of San Blas, in Nayarit. The river is viewed by some sources as a continuation of the Lerma River, which flows into Lake Chapala.