**1. Introduction**

A river needs to be understood as part of a changing system, so it cannot be studied in isolation. In this way, the benefits throughout the basin at different levels (high, middle, and low) and scales can be observed, which will favor the development of social and ecosystem services. However, to achieve comprehensive management of a basin, it is important to understand some of the main obstacles to river management, such as the changes that have originated through several actions, such as land use management (changes of coverage and use), climate change, development of aquatic

resources and expansion of the industrial sector, dams, among others. These actions have considered changes in the runoff pattern, the quality of the discharges to the river, the size distribution, and the load of transported sediments, as well as changes in the structure of fluvial and riparian micro-habitats.

Hydrological processes are governed by a large number of biophysical and climatic variables. No matter how small, any change made to sensitive and highly vulnerable ecosystems generates large changes. Thus, when the change is not so small and considers the installation of a dam, there is a modification that alters the energy flows, such as the precipitation and temperature, and, in consequence, the evapotranspiration (ET), groundwater recharge, runoff, and water bodies storage [1, 2]. This implies that the cause/effect relationship of the realized changes will have significant environmental implications, such as the loss of biodiversity, alterations in hydrological processes, and land degradation. When it comes to natural or artificial reservoirs, runoff can be attenuated or restricted, although secondary runoff associated with overflows can even occur, changing the ecosystem upstream and downstream in the river [3]. The main responsible for this type of change is associated with humans and their productive activities [2], including river regulation and global warming change, which are the main drivers of changes in the flow regime of rivers identified by [4]. In particular, a dam that, on one side, it will provide several benefits to people, such as diversion of water to agricultural or urban areas, electricity production, aid in navigation, and flood control [5, 6]. However, on the other side, there are also negative changes affecting the estuarine and coastal ecology, which has resulted, in many places, in the loss of habitats such as wetlands. In particular, vegetative populations, such as riparian or coastal vegetation (e.g., mangroves), show a structure defined by the availability of freshwater, nutrient recycling, tidal flows, frequency of flood periods, physical characteristics of sediments, and water chemistry [7–9]. As these ecosystems could be highly affected by the changes in the rivers and their plains, a geomorphological analysis is necessary, and it includes the vegetation and environment, as formations of geoforms making the system highly unstable). In general, the installation of dams is associated with the loss of natural structures, changes in the grain size distribution (structure and substrate of the river bed), and interruption of the sediment transport of the river bed (continuity of the river). Thus, it is important to consider that inducing a discontinuity in longitudinal migration will likely lead to fragmentation of the aquatic fauna. Also, special attention needs to be made to the hydrology alterations caused by river regulation and climate change. The continued temperature increase, as well as the reduction of water supply (precipitation), have the potential to modify the timing and magnitude of a river flow [10, 11]. Duan and Cai [12] noticed that global warming had affected the snow accumulation in winter; and its melting in spring, resulting in short timing with peak runoff and the increment of discharges earlier in the water year; thus, there are changes in the annual flow [13]. For instance, Duan and Cai [12] observed that flood risk was reduced in cold watersheds corresponding to high latitudes and altitudes. In contrast, an increase in flood risk was registered by Allamano et al. [14] due to the increase in temperature and precipitation intensity. It will be common for rain-dominant basins to present floods related to storms in the wet season [15]. Precipitation is characterized by frequency and duration, which defines its magnitude, as well as soil moisture. Thus, a reduction/increment of high flow magnitude is significantly connected to changes in precipitation characteristics, climate warming, and the spatial heterogeneity of the terrain [12]. Maskey et al. [4] pointed out that there is no single climate model that explains the behavior of the hydrological processes. Still, it is necessary to understand

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

both impacts and effects caused by the water bodies to set realistic managed environmental flows that cope with their ecological goals. Although, Maskey et al. [4] found that "the flow regimes downstream of a dam are likely more altered by reservoir operations than by climate change." Also, an intermittent flow created, as a result of the reservoir operations, affects the static instream flow requirements that might impact the movement, establishment, and environmental signals for aquatic life.

#### **1.1 Tidally-driven connectivity**

Streams, rivers, lakes, wetlands, and marshes are aquatic ecosystems that interact because of their ability to import and export material and energy altering the fluxes of these materials. These interactions require connectivity, where various transport mechanisms in a heterogeneous landscape define the degree to which components of a river system are joined or connected to other water bodies [16, 17]. Connectivity between freshwater and marine aquatic habitats offers benefits, such as migration of coastal habitats and species, protection of inland habitats against storms, waves, and seawater, food production, biodiversity protection, flood protection, and erosion control. In general, connectivity in these ecosystems is defined by characteristics of the physical landscape, climate, biota, and human impacts. The last one has partially restricted or completely obstructed tidally-driven aquatic habitats in coastal areas due to human developments (reclamation, species invasion, and environmental pollution), and infrastructure [18]. Thus, many coastal habitats have been disappearing because there is not enough space for inland movements. It is not only a longitudinal consideration, but it also requires studying the lateral and vertical movement since the first links ditches and floodplains, and the second the surface and subsurface [18]. It is important to contemplate the three of them in order to achieve good projects of maintenance or adaptation of these areas. In addition, it is necessary to consider that wetlands in riparian or floodplain areas can have bidirectional lateral hydrologic flows, whereas the type of connection between them only influences wetlands in non-floodplain areas. Also, it is necessary to consider whether vertical movement corresponds to an expansion or contraction of the river network since each one can affect the duration and timing of flow in this network [16].

An interesting aspect related to rivers is the transport of nutrients, sediments, chemicals, organic particles, microbes, detritus of various size classes, and living organisms downstream through wetlands, deltas, estuaries, and other downstream systems. Leibowitz et al. [16] identified the proportion of the material from (or reduced by) streams and wetlands, the residence time of the material in the downstream water; and the relative importance of the material to river function or ecosystem services as the main factor that influences at the material and energy fluxes from streams, and wetlands downstream. As connectivity can be defined as the degree to which components of a system are connected and interact through various transport mechanisms, it varies over time and space, and there are several methods to characterize or quantify it at the watershed scale, such as field hydrological monitoring, hydrological models, connectivity index, remote sensing approach (e.g., LIDAR data or aerial imagery) and graph theory [18]. Park and Latrubesse [19] used remote sensing to map the seasonal water extent and quality variabilities. Also, they identified channel-floodplain connection thresholds and validated results using field measurements. They confirmed that hydrological connectivity processes happen mainly through floodplain channels having specific river level thresholds over space and time characterized by different recharging conditions (through sequential river pulses), water residence, and recessional

periods. In addition, Freeman et al. [20] identified that changes upstream of a river affect directly downstream ecosystems, which are also associated with sea conditions, such as flooding, frequency by the tidal and cumulative flooding time. The last two are also responsible for the ecological structure and functions of coastal ecosystems, such as estuaries, mangroves, and tidal flats [21]. Thus, the main goal of this research was to quantify the impact of a dam operation in both river and marshland systems as a function of their connectivity by looking at its spatial (part of the basin) and temporal (dry and wet seasons) variations. For this, the hydrology was modeled for a period of discharges to evaluate the magnitudes of the flows, with or without the dam. This allowed identifying the modification of the hydrodynamic regime between the sea and the lagoon system during the dry and wet seasons.
