Inthis paper, water samples from 29 (twenty nine) tributaries of the Rio Negro went analysed and compared according to their physico-chemical qualities. The majority of the tributaries were very poor in dissolved minerals and highly acidic. Significant relationship were found between the level of colored organic substances ans several chemical parameters. The left bank afluents, originating form the mountains in the North, had higher nutrient levels. While the right bank afluents which, in their majority, originates in the "terras alagadiças" (swampy lands) and in "campinas" (podzol soils), were considered extremely poor. In general, physical, chemical, and biological conditions affect the nitrogen values. The recent fertilization of flooded areas and the appearance of aquatic macrophytes in the Lower Negro are discussed.