Tietê River

Each year around Tietê River Day on September 22, the SOS Mata Atlântica Foundation publishes Observando o Tietê, a water-quality study built from monitoring points across the basin. The 2025 edition covered 55 points on 41 rivers, and only one point, 1.8 percent, was rated good. The stretch of the main river with water unsuitable for multiple uses shrank from 207 kilometers in 2024 to 174 kilometers in 2025, a 15.9 percent reduction.

A separate 2025 expedition from the river's source to its mouth sampled 14 points. The foundation reports microplastics at every point, 25 types of pesticides, and 16 pharmaceutical and illicit-drug substances, and concludes that no stretch of the river is free of contamination. Reporting on data from Cetesb, the São Paulo state environmental agency, says the organic load leaving the São Paulo metropolitan region fell from 218 tons per day in 2024 to 154 tons per day in 2026, so the picture is mixed.

SOS Mata Atlântica is a Brazilian foundation focused on the Atlantic Forest and its water. Through its Observando os Rios program, which began in 1993 as Observando o Tietê, volunteers measure river water quality monthly with a kit the foundation developed. Its 2025 Tietê study draws on 46 volunteer groups in 24 municipalities. It is the organization most directly tied to this river in our research.

Instituto Trata Brasil, a civil society institute, works on sanitation across Brazil and publishes research and an annual sanitation ranking of cities. Instituto Limpa Brasil, a non-profit, coordinates World Cleanup Day in Brazil and runs waste and cleanup campaigns. Their sources do not tie either one to the Tietê, so they appear as country-level organizations to explore.