A state estimation method for two-dimensional shallow water equations (SWE) in rivers using Lagrangian drifter positions as measurements is proposed. Lagrangian drifters are sensors moving with the flow and reporting their location. The aim of this method is to compensate for the lack of accurate information about boundary conditions. The drifters move with the local flow and report their positions. Thus, they provide additional information about the state of the river compared to a case in which, for example, only a computational model is used to describe the river flow. In this work, the...