Foam flows over the surface of the Tiete River in Salto, Sao Paulo state, Brazil, Wednesday, Oct. 13, 2021. The foam is caused mostly by untreated household runoff from nearby Sao Paulo, the biggest city in Brazil, according to Malu Ribeiro, water-program coordinator for the local environmental nonprofit SOS Mata Atlântica. (AP Photo/Marcelo Chello)