nitrogen and phosphorus are primary nutrients, common constituents of fertilizers, detergents, and domestic and industrial wastewater discharges. these unloadings to rivers and subsequently to lake maracaibo cause nutrient and mineral enrichment, producing a secondary eutrophication on rivers and detained waters at deltas. lake maracaibo system is a basin of extraordinary importance because of its energetic resources, biodiversity and the wide number of aquatic organisms, which represents an important economic source for the country. however, activities being developed on the basin such as agricultural, cattle raising, mining and industrials, have modified the physical, chemical and biological characteristics of both water and soil generating an accelerated eutrophication process, as well as, the appearance of dense populations of the aquatic weed lemna obscura, generating social, environmental and economic impacts. the following river basins were evaluated: limón, palmar, santa ana, catatumbo, escalante, chama, motatán, misoa, machango and pueblo viejo. these rivers contribute with 80% of the freshwater to the lake maracaibo system, being the catatumbo river contribution 60% of the volume. total nitrogen and total phosphorus average of rivers that drain to the lake, was 1.20 and 0.84 mgl-1, respectively, contributing significantly with the process of eutrophication, that combined with the expansion of the agricultural borders, have caused deforestation, erosion and contamination, altering landscape and water availability, as well as an important increase of nutrients in lake maracaibo.