Some of the most populous ones are the Guarani, Tikuna, makuxi. The majority of the tribes were assimilated in 2019-2010. The country is going through a fight in the courts with president Bolsonaro on the legitimacy of some of the smaller tribes, and the land they own.
A glacial valley cross-sectional shape is a U shape whereas a stream valley is usually a V shape. The U shape comes from the scouring action of the alpine glaciers which carries rocks with it which can abrade the walls of the valley and the glacier occupies the whole valley so scours out such a shape whereas the stream cuts down and water washes in from the sides of the valley into the stream or river creating the V shape.
I believe it would be Omo Kibish, southern Ethiopia.
