Extreme climatic events may be translated into a higher frequency of both dry and wet years. Frequent droughts pose a challenge to water supply in terms of both quantity and quality. In order to cope with this, there has to be a documentation on the concentration of chemicals in water during such events. The southeast region of Brazil experienced a major drought event in 2014 leading to number of social, economic, and environmental impacts. The objective of the present paper is to understand the effect of an extreme drought on dissolved organic and inorganic carbon concentrations of the Piracicabal river, which is under high human pressure. Concentrations of both dissolved organic (DOC) and inorganic carbon (DIC) were monitored daily, for 30 days, during a dry month in 2014. DOC values found here were at least twice those observed in the river in non-extreme conditions. Rain events within this drought led to significant differences in discharge, DOC, and DIC. We built a dilution-concentration model which described an exponential relationship between discharge and both carbon forms. This indicated that drastic changes in concentrations of DOC and DIC are expected during such events. Our results may also apply to a number of regions in the world especially those of the developing countries where rivers are highly subjected to sewage impact.