The Pastaza (formerly known as the Sumatara) is a large tributary to the Marañón River in the northwestern Amazon Basin of South America.
It has its headwaters in the Ecuadorian province of Cotapaxi, flowing off of the northwestern slopes of the volcano Cotopaxi and known as the Patate River. The Patate flows south and in Tungurahua Province it is joined by the Chambo River just upstream from the town of Baños de Agua Santa just north of the volcano Mount Tungurahua and becomes the Pastaza. Seven kilometers east of Banos, it is dammed for the Agoyan hydroelectic project, which has created a silty lagoon by the village of La Cieniga. The Agoyan dam was placed in that location specifically to leave the famous Falls of Agoyan, about 5 km further downstream, intact. After the waterfall the river enters a gorge where there is very fast whitewater with class-4 rapids; it is often used for whitewater rafting although it is not considered to be of the same quality as the Tena River and is therefore less popular for the sport.