**Abstract**

When the river supplies water to vulnerable environments, such as marshlands, it is vital to establish the expected impact mostly under a changing climate, and moreover, if a dam is being projected to solve energy demands. Soil characteristics, specifically sediment composition, are exposed to changes that modify this type of ecosystem and are rarely investigated. For this, a discharge period for an average historical year was analyzed to evaluate the magnitudes of the flows, with or without a dam. Also, it helped to identify the modification of the hydrodynamic regime between the sea and the lagoon system, particularly during the dry season but also checking the behavior in the rainy season. Results showed that the main problem with the construction of the dam on the San Pedro-Mezquital river would be the effect of a controlled flow that reaches the wetlands of the alluvial plains, affecting the sediment load in the estuarine and coastal ecology. However, after a readjustment period, the dam neither significantly changes the previous flood conditions of the coastal plain nor the sediment load will be a problem. However, if an additional sediment load is required to maintain the coastal microhabitats, there are different ways to provide it.

**Keywords:** marshlands, river connectivity, wetlands, Marismas Nacionales, dam
