Almost all of the rivers in Ecuador rise in the Sierra region and flow east toward the Amazon River or west toward the Pacific Ocean. The rivers rise from snowmelt at the edges of the snowcapped peaks or from the abundant precipitation that falls at higher elevations. In the Sierra region, the streams and rivers are narrow and flow rapidly over precipitous slopes. Rivers may slow and widen as they cross the hoyas yet become rapid again as they flow from the heights of the Andes to the lower elevations of the other regions. The highland rivers broaden as they enter the more level areas of the Costa and the Oriente.

In the Costa region, the Costa Externa has mostly intermittent rivers that are fed by constant rains from December through May and become empty riverbeds during the dry season. The few exceptions are the longer, perennial rivers that flow throughout the Costa Externa from the Costa Internal and the Sierra on their way to the Pacific Ocean. The Costa Internal, by contrast, is crossed by perennial rivers that may flood during the rainy season, sometimes forming swamps.

The Guayas River system, which flows southward to the Gulf of Guayaquil, constitutes the most important of the drainage systems in the Costa Internal. The Guayas River Basin, including land drained by its tributaries, is 40,000 square kilometers in area. The sixty-kilometer-long Guayas River forms just north of Guayaquil out of the confluence of the Babahoyo and Daule rivers. Briefly constricted at Guayaquil by hills, the Guayas widens south of the city and flows through a deltaic network of small islands and channels. At its mouth, the river forms a broad estuary with two channels around Puná Island, the deeper of which is used for navigation.

The second major Costa river system--the Esmeraldas--rises in the Hoya de Quito in the Sierra as the Guayllabamba River and flows westward to empty into the Pacific Ocean near the city of Esmeraldas. The Esmeraldas River is 320 kilometers long and has a 20,000-square-kilometer drainage basin.

Major rivers in the Oriente include the Pastaza, Napo, and Putumayo. The Pastaza is formed by the confluence of the Chambo and the Patate rivers, both of which rise in the Sierra. The Pastaza includes the Agoyan waterfall, which at sixty-one meters is the highest waterfall in Ecuador. The Napo rises near Mount Cotopaxi and is the major river used for transport in the Eastern lowlands. The Napo ranges in width from 500 to 1,800 meters. In its upper reaches, the Napo flows rapidly until the confluence with one of its major tributaries, the Coca River, where it slows and levels off. The Putumayo forms part of the border with Colombia. All of these rivers flow into the Amazon River. The Galápagos Islands have no significant rivers. Several of the larger islands, however, have freshwater springs.