The acquisition of good hydrologic information is an important issue in water management since it is the basis of decisions concerning the allocation of water resources to different users. However, sufficient data are often not available to describe the behaviour of such systems, especially in developing countries, where monitoring networks are inappropriately designed, poorly operated or are inadequate. Therefore, it is of interest to design and evaluate efficient monitoring networks. This paper presents two methodologies to design discharge monitoring networks in rivers using the concepts of Information Theory. The first methodology considers the optimization of Information Theory quantities and the second considers a new method that is based on ranking Information Theory quantities with different possible monitor combinations. The methodologies are tested for the Magdalena River in Colombia, in which the existing monitoring network is also assessed. In addition, the use of monitors at tributaries is explored. It is demonstrated that the ranking method is a promising way of finding the extremes of Pareto fronts generated during multi-objective optimization processes and that better (more informative and less redundant) monitoring network configurations can be found for the Magdalena River.