This study presents an analysis of the dissolved oxygen (DO), temperature (T) and their behavior in the Medellin River, the more important body of water that forms the Aburra Valley in Colombia. Some correlations are established between these two parameters and the expected behavior for a clean water body at a given atmospheric temperature and pressure. The analysis was based on data obtained in the Phase II of the Red Rio project that was developed by a group of universities for the local environmental authority to study the river. After an extensive analysis of multiple OD and T data that was taken at 20 sampling stations, this review presents a water environmental quality index easy to calculate, based on the differences between the T and OD experimental curves and their expected behavior for an uncontaminated stream . The results show, in a very clear and didactic way, how the river behaves across the 20 sampling stations. We propose the use of this index as a practical tool to maintain a continuous monitoring of the environmental performance of this river or other watercourses.