**2. Study area**

The study area is in the state of Nayarit, which has the largest number of mangroves together with the state of Sinaloa, sharing the Teacapán-Agua Brava-Marismas Nacionales lagoon system [22]. The rivers that drain the system are Cañas, Acaponeta, Rosamorada, Bejuco, San Pedro, and Santiago, with 161,515 km2 . This study area is located in the physiographic province of the coastal plain of the Pacific, in the sub-province of the delta of Santiago. Along the river, lands of the deltaic landscape system characterized by marshes with coastal lagoons and parallel bars of old coastlines can be found (**Figure 1**). The soil types correspond to fluvisols, cambrisols, and feozem with some gleysols. The feozem is well-drained, fertile, and productive soils. Thus, these plains are excellent for agriculture. In general, more of the landscapes are converted into agricultural lands.

#### **Figure 1.**

*General scheme of the San Pedro river from the Las Cruces Hydroelectric Project (PHLC) to the Marismas Nacionales zone.*

#### *Interconnection among River Flow Levels, Sediments Loads and Tides Conditions and Its Effect… DOI: http://dx.doi.org/10.5772/intechopen.109175*

Climate is sub-humid and warm with summer rainfall and semi-warm sub-humid. The average annual precipitation is between 1000 and 1500 mm, reaching more than 2000 mm in September and October. However, rainfall is also associated with the presence of the Mexican monsoon and the occurrence of hurricanes and tropical storms, which may appear between June to October each year. Vegetation includes agriculture occupying 10.67% of the alluvial plains with a low slope (less than 1 m) of the high plateau with deep and moderately deep soils. On both river banks, two irrigation districts (DR) are developed: 052 and 043 are at the present.

Bojórquez et al. [23] identified that the deltaic plains are made up of alluvial material. The presence of dynamic processes is manifested in erosion (in the channel) and accumulation along the flood plain (terraces and dikes). This type of geomorphological landscape in the lower part of the San Pedro-Mezquital river basin, dividing it into (a) intermediate fluvial plain (second level of a fluvial terrace at heights of 5–10 m, from which the rivers overflow in extraordinary flows), (b) low plain with fluvio-marine influence, and (c) fluvial flood plain. In the latter, it is found in the current riverbeds with fluvisols subjected to the action of the systematic flooding of the Santiago, San Pedro, and Acaponeta rivers, where a rejuvenation process is active, which is manifested by the presence of fluvic material in the first 50 cm at the thick soil. In the Marismas Nacionales (MaNas, National Marshes) region, there are few evolved mineral soils of unconsolidated colluvial-marine contribution with moderate erodibility and salinization or sodification [23].

The MaNas is estuary's most important wetlands system, where the mangrove is the dominant vegetation (15–20% of the total of this valuable ecosystem) with species reaching up to 20 or 30 m tall and occupying almost 175,000 hectares [24]. Other vegetation types are halophytic and tular vegetation; the distributions of mangroves and halophytic vegetation clearly respond to a hydraulic regime of flooding. However, these areas have been affected by human beings who have modified the riverbed not only in the mangrove area but also upstream with deforestation and the creation of dams or canals to guarantee agricultural activities. Blanco y Correa et al. [24] noted that from the total area of the estuary, around 135,000 ha (77%) suffered from tidal disturbance; between 5000 and 10,000 ha (from 2.8 to 5.5%) have disappeared under the sea due to coastal erosion, and 5000 ha (2.8%) represented effects from the hydro-sedimentary imbalance of its river. Thus, more than 15,000 ha (8.86%) have an accumulated and synergistic environmental deterioration of at least four to six decades of interaction with different factors, such as poor water, sediment quantity, and quality.

As the MaNas area is important for its high biodiversity and ecosystem services, it is required to take care of its mangroves and, in general, of the entire wetland system. In this way, the real impact that river systems have in the face of natural and anthropic changes, such as altering rivers' capacity to recover or presenting different losses in the hydrological basin needs to be considered. Some actions to protect these ecosystems are the reduction of the possible erosion of beaches, eutrophication of bodies of water, loss of habitat, loss of soil fertility, and reducing the vulnerability to storms or tidal waves, among others.

It is important to recognize that in the MaNas area, the contribution of water is associated with the rain and runoff from the San Pedro river and the effect of the underground movement of water, which means that it can move from neighboring lateral areas into and out of the areas bounded by the watershed. In other words, the hydraulic behavior of the MaNas zone owns complex hydrological connectivity due to the existing interaction between freshwater and marine water. There are associations

between the periods of sedimentation and marine regression, which largely affects the sediments' character (differentiation of mechanical particles and/or carbon content in the thickness of the soil profile), the spatial distribution of salinity, and the predominant type of vegetation. This makes MaNas different from the rest of the basin, being the movement of subsurface water decisive. In fact, the drainage patterns of the lower courses of the San Pedro-Mezquital river changed due to the deposition of sediments and the construction of protection levees on the riverbanks. Although the dam will signify greater flood control, at the same time, it will introduce changes in the coastline, intensifying the erosive processes of channels, and the accumulation of sediments in the coastal lagoons.
