The Amazon Basin is composed of a mosaic of interconnected freshwater and terrestrial ecosystems. The complexity of these connections depends not only on the hydrological patterns but also on the geomorphic features in the floodplain, creating a heterogeneous network of wetlands, lakes, channels, and alluvial deposits that varies in space and time (Alsdorf, 2003; Park and Latrubesse, 2017). Seasonally based flooding patterns of both hydrological and hydraulic regimes also create a diverse mosaic of vegetation and habitats, supporting the functioning of ecosystems. As a result of the constant development of the floodplains caused by seasonal alluvial flows, previous studies suggest that the diversity of the vegetation from different ages generated the high biological diversity in the Amazon rainforest (Salo et al., 1986). Therefore, dynamic flow regimes establish critical interactions between the main channel and adjacent floodplain habitats, such as “tipishcas” or oxbow lakes, thereby providing fish and other aquatic species access to shelter, food, and spawning grounds (Junk et al., 1989; Couto et al., 2017; Virgilio et al., 2020). According to Castello et al. (2013), this dynamic interaction explains the high ecological value of the Amazonian ecosystems but also makes them more sensitive to anthropogenic pressures. Flood frequency and duration of inundation are relevant to characterize the hydrological connectivity and the ecological features of Amazonian ecosystems (Wilson et al., 2007; Castello et al., 2013; Reis et al., 2019). Reis et al. (2019) also indicate that the predominance of local rainfall in flooding patterns of complex wetlands makes them more sensitive to climate change and external pressures at a regional scale but less vulnerable to alterations in the local hydrology of the river. However, this is the opposite for systems dominated by floodplain inundation that depend on the overflow from large rivers and lateral connectivity. Moreover, Arantes et al. (2019) demonstrated that the biomass distribution of fish functional diversity in floodplains is associated with the vegetation cover, resulting that biomass being most significant in areas of the flooded forest when compared to other habitats.

The Peruvian Amazon is one of the regions that harbor the largest biodiversity on the planet, with a world record for biodiversity per unit area (OTCA, 2009). Some theories explain that this biological value depends on the correlations of ecosystems with the dynamics of rivers because it is related to flooding patterns (Ferreira and Stohlgren, 1999), the geomorphology of the riverbed, the altitudinal climatic changes, and the influence of the upper part on the river channel (Naiman et al., 1993). A dynamic fluvial system helps to modify the flora, unlike a more stable environment where the time scale of these processes is much longer. Kalliola et al. (1992); Kalliola et al. (1993a); Kalliola et al. (1993b) and Puhakka et al. (1992) explained that the vegetation patterns and the diversity of the species are usually controlled by other environmental factors, such as 1) the dynamics of the river that defines the type of vegetation and habitats; 2) the influence of the change of season in the plain alluvial that generates regions of different elevations, age, and soil properties (deposition processes along the point bars, ridges, depressions, flooded basins, and abandoned channels); and 3) the alluvial processes that create new habitats, modify existing ones and produce a diversity of vegetation. Additionally, the seasonal intermittency of discharge into rivers produces variations in water cycles, which are associated with the frequency of floods, erosion of riverbanks, changes in fishing activity, and changes in the planimetric and altimetric patterns of rivers (Salonen et al., 2012; Abad et al., 2013).

The primary objective of this study was to explore the ancient and modern river geomorphology across the Pacaya-Samiria wetland system (the second largest protected area in Peru), considering the high geomorphological complexity observed in this region (Dumont, 1991; Dumont, 1992). Based on it, the second objective was to relate the landscape dynamism to water fluxes, peat vegetation, and fish extraction. This study hypothesizes that the characterization of wetland landscapes will help understand the gradients in habitats and support decision-making in conservation and planning strategies for Amazonian wetlands.

The Pacaya Samiria National Reserve (PSNR) is in the west portion of the Amazon basin (Figure 1A), a region where the Amazon River is born due to the confluence of the Ucayali (meandering) and Marañon (anabranching) rivers (Garcia et al., 2022; Guerrero et al., 2022), as illustrated in Figure 1A. The PSNR is a protected Ramsar Site of 20,800 km² (one of the largest flooded forests within the Amazon basin) composed of a tropical seasonally flooded forest dominated by rainfall and upland run-off (Castello et al., 2013), where biodiversity indicators (e.g., terrestrial mammal richness, tree a-diversity) are higher compared to other regions in the Amazon basin (Hoorn et al., 2010).

From the geological point of view, the Pacaya-Samiria (P-S) wetland develops in the subsiding Ucamara depression of the Marañon Basin in the Peruvian Andean forelands. It is a very flat subsiding area of 330,000 km², consisting of a complex network of rivers and permanent and semi-permanent swamps and lakes, and paleochannels. The depression is approximately delimited on the north by the Marañon River and to the southern and eastern borders by morphostructural boundaries and the Ucayali River. The Ucamara depression was an active sediment sink that was filled up by with sediment sourced from the Andes by avulsed alluvial belts of the Ucayali and Marañon (Dumont, 1992; Latrubesse, 2015). The thickness of the Quaternary deposits is unknown, but more than 15 m are exposed in bank outcrops of Quaternary terraces in the Ucayali River. Figure 1B shows that the P-S wetland overlies with the Chambira (continental) formation in the Ucayali Basin of Peru, to later overlap with the Solimões (previously known as Pebas) formation in the confluence between the Ucayali and Marañon Rivers (da Silva-Caminha et al., 2020; Guerrero et al., 2022).

Figure 1C shows that the P-S wetland has a low-water stage from October to November, whereas flooding (high-water stage) occurs between May and July (Hess et al., 2015; Reis et al., 2019). In this ecosystem, bordering rivers are narrow (Reis et al., 2019) and there is a direct correlation between flooded area percentage (open water, inundated non-forest, and inundated forest) (Jensen et al., 2018) and river level measurements. As illustrated in Figures 1C, D, the P-S wetland cover is composed of late-successional vegetation, such as forests and woodlands, that are inundated most of the year. During the high-water stage, areas along the streams (bare soil) inundate, allowing early successional vegetation, such as aquatic macrophytes, to grow.

Figure 1E shows that the P-S wetland includes different river systems: self-formed meandering (Ucayali, Huallaga, Tigre, Tapiche, Puinahua), underfit-scavenger-formed meandering (Yanayacu, Pacaya, Samiria), and anabranching (Marañon, Amazonas). The Huallaga, Ucayali, and Tapiche Rivers enter the P-S wetland with an increase in the valley angle―50, 55, and 70°, respectively, thus avulsion processes and meandering migration rates are enhanced (Abad et al., 2022a; Abad et al., 2022b). As observed in Figure 1E, there is a reduction in valley angle, promoting the birthplace of the Amazon River as the confluence of the Marañon and Ucayali Rivers (Garcia et al., 2022; Guerrero et al., 2022).

Previous studies have described the complex geomorphology within the P-S wetland, composed of interconnected rivers and swamps (Dumont and Garcia, 1991; Kalliola et al., 1992). The Marañon, the Ucayali, and the Tapiche are white-water rivers (high concentrations of solutes and sediment) that flow from west to east and south to north, respectively, while the Pacaya, Yanayacu and the Samiria are black-water rivers (high concentration of dissolved organic carbon) with similar geomorphic patterns that resulted from the accumulation of rainfall and flooding on fossil fluvial landforms (Dumont 1991; Duncan and Fernandes, 2010). Dumont. (1991) described that the composition of fluvial landforms along the Pacaya River is similar to those Ucayali River present, thus suggesting that the Ucayali River has flowed northward forming the Pacaya River valley to shift southeastward later. Likewise, the Samiria River presents similar fluvial landforms to the Ucayali River, but that are more preserved than Pacaya’s. These landforms are also surrounded by the Marañon River, indicating that Samiria was formerly connected to the Marañon River. Dumont. (1991) and Räsänen et al. (1992) indicated that the Samiria, Pacaya, and Tapiche are then underfit-scavenger rivers that rely on relic belts left by the Ucayali River. Additionally, Dumont. (1991) suggested that the Ucayali and Marañon Rivers have been joined on the western side of the P-S wetland and later the junction migrated eastward due to avulsion shifting dynamics.

Puhakka et al. (1992) described that sediment deposited in adjacent bars in meandering rivers tends to inundate during the high-flow season then allowing plant colonization in the following months. Herein, long-term aggradation and deposition of meandering river sediments result in alternated vegetation mosaics (Kalliola et al., 1992). Likewise, other colonized areas include poorly drained swales between the ridges. In meandering systems, such as the Ucayali and Huallaga Rivers, banks in chute islands (result from chute cutoff) are prone to erosion thus limiting vegetation succession, whereas river margin areas are more stable and can extend many kilometers downstream allowing vegetation grows in aged zones. Studies of vegetational successions in tropical and subtropical anabranching rivers are less known (Wittman et al., 2006; Peixoto et al., 2009; Montero and Latrubesse, 2013; Stevaux et al., 2013; Marchetti et al., 2013; Marchetti et al., 2020; among others). In anabranching systems, such as the Marañon River, braided channels are constantly dividing and rejoining; thereby, developing an unstable system composed of young successional vegetation. Additionally, Draper et al. (2014), Lähteenoja et al. (2012), and Lähteenoja and Page. (2011) suggested linkages between vegetation history with carbon stocks in peatland environments across the P-S wetland in the Pastaza-Marañon foreland. Figure 1F shows larger open peatland areas across the Ucayali River, and pole forests and palm swamps along the Marañon River. High carbon density has been found in below-ground pole forests, followed by palm swamps and pole forests (Draper et al., 2014), which suggests that the Marañon valley has greater carbon stocks than the Ucayali.

Gradients in land cover, because of inundation timing and consequent vegetation succession, are often associated with animal functional diversity and biomass in Amazonian floodplains (Arantes et al., 2019). The interconnected system between the Marañon and Ucayali Rivers combined with the exchange of water and sediment (Anderson et al., 2018) supports a high alpha and beta diversity of a myriad of fish species due to the habitat specialization and geographic isolation conditions (Albert et al., 2011; Aldea-Guevara et al., 2013). In the Marañon and Ucayali basins, more than 1,300 fish species have been estimated (Anderson et al., 2018; Latrubesse et al., 2021). The Ucayali basin encompasses more than 650 species of fish, 90 to 110 species of floodplain trees and more than 80 species of floodplain birds, whilst the Marañon basin sustains around 700 species of fish, the highest tree diversity among Amazon sub-basins, and more than 50% of flooded-habitat birds identified in the Amazon (Latrubesse et al., 2021). In addition, the Amazon basin reaches the highest diversity in the Marañon basin and Andean-foreland rivers have the highest tree diversity compared to any floodplain forests on Earth, including up to 30% endemic tree species (Wittmann et al., 2013).

Figure 1G shows about 1,112 fish extraction zones georeferenced across the Loreto region, of which 28% are in the PSNR along the Marañon, Huallaga, and Ucayali rivers (Wildlife Conservation Society, 2022). More than 60 billion kilograms were extracted in a 5-year period (from 2015 to 2017) in Loreto, and only 40% of the total fish extraction was extracted from the Ucayali River. Moreover, Figure 1F indicates a higher concentration of fishing areas near the San Lorenzo community (Marañon) but a lower fish extraction capacity. In contrast, locations near the Requena community (Ucayali) show a higher fish extraction capacity but a lower concentration of fishing zones. Note that this information was derived from the landings of commercial fishing registered in the main ports. Although the Regional Directorate of Production of Loreto (Dirección Regional de la Producción de Loreto, in Spanish) collected all data, there may be underestimates of the total fish volume of landings because not all ports have enough personnel to collect that information (Wildlife Conservation Society, 2022).

The Science Panel for the Amazon. (2021) described that the biodiversity of terrestrial and freshwater ecosystems is under the constant pressure from socioeconomic activities, such as deforestation for agricultural development and mining (Volckhausen, 2020), pollution (especially from constant oil spills; Sierra, 2020), climate change and the development of river-related infrastructure (dams, roads, waterways; Gonzales, 2019; Sierra, 2019), affecting indigenous lands and protected areas as well. Human-induced disturbances are generally associated with habitat degradation, loss of species, and, consequently, loss of ecosystem services. For instance, sedimentation (excessive deposition of fine particles) may cause impacts on community composition and species losses, thus affecting energy flow and trophic diversity (Osmundson et al., 2002; Burdon et al., 2013; Burdon et al., 2020). In the Amazon region, little is known about how ecosystem components (e.g., plant-animal interactions, habitat distribution) might respond to these pressures. The lack of adequate management strategies is affecting the management of not only aquatic habitats but also carbon-rich ecosystems and terrestrial biodiversity (Castello et al., 2013).

Previous studies indicates that fisheries productivity in the Amazon is linked to seasonal water level changes which conditions feeding and spawning opportunities (Junk et al., 1989; Isaac et al., 2016). Currently, population declines are driven by habitat fragmentation, overharvesting, barriers to migration, and climate change (Duponchelle et al., 2021). In general, Anderson et al. (2018), Gerstner et al. (2006) and Kirkland et al. (2020) have discussed how pressures (e.g., infrastructure) on rivers can affect aquatic diversity (fish richness and population; Wildlife Conservation Society, 2020). Thus, it is very relevant to incorporate the link between river geomorphology (from water to sediments to erosion/depositional patterns) and biodiversity, not only for baseline studies but for predicting future impacts, thus the science and information can be of support for better decision-making.

A major concern in the Amazon are the impacts produced by the existing dams and the potential impact of tens of planned dams. River disturbance by dams may affect sediment supplies river channel and floodplain dynamics and can decrease or even suppress ecological connectivity among populations of aquatic organisms and organisms that depend on flood seasonality (Finer and Jenkins, 2012; Flecker et al., 2022). Latrubesse et al. (2017) provided an analysis of the potential irreparable consequences for Amazon rivers that can be expected at different scales and sub-basins by more than 400 dam that exist already or are under consideration for construction, especially along the upper Marañón river (Flecker et al., 2022). The analysis to vulnerability by dams by using the Dam Environmental Vulnerability Index (DEVI) points that the Marañon and Ucayali are the most vulnerable Andean Rivers. Regarding flow regulation and decrease of sediment load, the Ucayali River, the most active river flowing through the P-S wetland, is the most sensitive Andean River to dam building (Latrubesse et al., 2017). Reductions of these factors by dam construction pose threats to wetland creation and maintenance. As discussed above, the anabranching Marañon River is less dynamic in terms of sediment regime and changing morphology but is critically threatened by a large number of planned and built dams in mountainous reaches of its main channel and it can decrease both the sediment supplies and inundation potential which maintain floodplain and the peripheral wetlands environments of PSNR. Furthermore, if sediment and water is reduced in an anabranching system, a transitional (from anabranching to meandering) condition could be triggered, therefore, promoting significant lateral migration of rivers (similar to the bifurcated channels in Figure 4C). The assessment of the potential impacts on the P-S wetlands by hydrophysical modifications of the Ucayali and Marañon by dams would need a priority. Both the Ucayali and Marañón rivers interact with the P-S wetland in terms of hydrology and geomorphology, proving the wetland with sediment and nutrient fluxes every year (hydrology dominated) and when there are bifurcations or significant planform modifications like the Puinahua River (geomorphology dominated), thus, the lateral connectivity of both systems is important for the sustainability of the biodiversity of the P-S wetland.

Likewise, this study provides indirect evidence that incorporating river dynamics is central to determine the distribution and to characterize the hydrophysical genesis and behavior of river wetlands and related peatland-type ecosystems. Figure 10 shows the overlap between the peat and non-peat-forming vegetation with the modern and ancient river dynamics in the P-S wetland unveiling vegetation patterns, composed of large areas of palm swamps. Palm swamps are nutrient-rich peatlands that resulted from a higher frequency of river avulsions and river dynamism (Draper et al., 2014), allowing oligotrophic conditions (low levels of nutrients) for certain aquatic species to spawn and feed (Lähteenoja and Page, 2011). Moreover, the current and past dynamics of the Ucayali River, evidenced by the complex network of paleochannels, support the development of early-successional vegetation.

As observed in Figure 10A, the recent low dynamism of the Marañon River (ancient and modern conditions) suggested appropriate conditions for pole forest and late-successional vegetation to develop, also allowing peat to accumulate and ombrotrophic condition (i.e., rainfall fed) to remain (Draper et al., 2014). On the other hand, in the bifurcation of the Ucayali River with the Puinahua River there are patches of open peatlands that are prone to frequent fluvial activity due to their high dynamics, as shown in Figures 10A, C. Large and dynamic rivers, such as the Ucayali River, then help maintain higher nutrient inputs to these areas, impeding to ombrotophic conditions and late-successional vegetation to develop (Dumont, 1991; Draper et al., 2014).

The hydrologic cycles in the Marañon and Ucayali Rivers are dominated by the precipitation patterns of the eastern Peruvian Mountains. Every year, these two rivers flood during the rainy season depositing nutrient-rich sediment into the flooded forests. Furthermore, the planform complex geomorphology between the Marañon and Ucayali rivers, showing numerous islands, oxbow lakes, and cutoffs within floodplains, has resulted in the creation of underfit-scavenger streams like Pacaya, Samiria and Tapiche rivers. In this research, modern and ancient signatures unveiled high river dynamics and shifting by the Ucayali River, evidencing frequent avulsions and erosional/depositional processes. The resultant environmental mosaic of the P-S wetland is the consequence of a palimpsest of relict geomorphological fluvial features that imprint spatial control to the hydrosedimentological routing and flooding processes in the wetlands. This investigation provides an integrative characterization of how fluvial shifting in the PSNR created suitable conditions and hydro-connectivity process to sustain both nutrient-poor and nutrient-rich environments, allowing different fish assemblages to spawn. The identification and characterization of present and paleo-fluvial landforms are then crucial for fishery management and carbon stock estimations.

During the high flow season, a considerable number of fluvial networks traverse the watershed between the Ucayali and Marañon rivers following ancient fluvial landforms (paleochannels). Annually, the flooded forests in the PSNR are enriched with nutrient-dense sediments generated by erosional processes upstream of the Marañon and Ucayali Rivers, which are later transported downstream and over the lowlands by the floods. Algae growth in the Amazon relies on the phosphorus and nitrogen conducted from the main rivers to the floodplains, and so the magnitude of the flood determines the number of algae produced and thus the number of fish that feed on these algae (Goulding, 1980). Additionally, these floods enable the lateral migration of fish and other aquatic life and the movement of nutrients from these rivers to one another or nearby streams. Therefore, the river dynamics associated with hydrogeomorphological events could favor the fragmentation, isolation, and dispersion of populations through changes in river courses (avulsions and captures phenomena) and disconnection and recapture of oxbow lakes from the main channel. The hydrogeomorphological events would favor allopatric speciation and then, during reconnections or course changes, it would lead to colonization of the sub-basins by neo-species.