Natural forest systems are potentially the best use of land for the protection of water resources, while agricultural practices tend to alter the physical, chemical and biological characteristics of water. For Brazil to consolidate the use of forests, especially riparian forests, as a management tool, it is still necessary to deepen the understanding of the specific influences of each land use in streams, as well as the effects of forests on the ecological functions of streams that determine the water quality. At the same time, it is important to understand the relationship between these forests and hydrologically sensitive areas (AHS). Thus, the objective of this project is to investigate the role of natural forests in hydrologically sensitive areas in controlling and maintaining the water quality of small-order tropical streams, particularly those that drain basins with intensive agriculture. To this end, three studies will be conducted concurrently in the watershed of the Corumbataí stream in the State of São Paulo. In the first one, we intend to analyze the influence of forest fragments that differ in size and successional stage on the water quality of streams draining pasture and sugarcane areas. In the second, the direct influence of riparian forests on the ecological functions of streams, such as aquatic metabolism, will be investigated, aiming to understand how agricultural practices in sugarcane fields may or may not be attenuated by the presence of riparian forest. Finally, it is intended to assess whether there is an overlap of these forests on the AHS, in order to understand whether the performance of ecosystem functions that determine water quality is dependent on the presence of forests on the AHS. These are expected to show that the densest and oldest forest remnants, and that overlap almost entirely within the AHS, can be powerful tools for managing the water quality of aquatic systems in regions with intensive agricultural activities.