The mouth of the Guadiaro River (SW Spain), oriented in a NE-SW direction, is often closed, resulting in environmental, social, and economic problems. An analysis of the factors influencing this phenomenon was performed using the Coastal Modeling System, a numerical model based on the mild slope equation (waves, currents, and sediment transport), and a coastal evolution study was performed comparing aerial photographs to the monitoring campaigns carried out in 2005 and 2006 (topographic surveys, current meters, tidal gauges, and water quality parameters). Global transport at the Spanish south Mediterranean coast occurs in a southwestern direction. However, the spit that closes the Guadiaro River grows in the opposite direction. The points and conditions where transport direction changes occur along the coastline were identified. The results, obtained from 200 different propagations, were consistent with field data, showing that northeastern littoral transport during easterly waves was sufficient to block the river mouth. Furthermore, significant differences in sediment transport patterns were found despite the existence of a fixed wave power direction. Through this research we also learned that use of wave propagation numerical models by nonexperts is prone to error, as small variations in input parameters can induce dramatically different results that may be contradictory in certain bathymetric configurations.