KEYWORDS
Hydrological Processes, Sediment Transport, Tidal Channels, Connectivity, Water Quality, Amazonian Estuaries.
1 INTRODUCTION
The Amazon River is the largest river in the world in terms of extension, area and discharge – its mean net discharge reaches 172,000 m³s-1 (Gallo and Vinzon, 2015) and represents approximately 16-20% of all continental freshwater received by oceans worldwide (Latrubesse et al., 2017; Ward et al., 2013; 2015). Its solid discharge represents approximately 600 to 800 x 106t.year-1 (Filizola and Guyot, 2009, 2011); maximum discharge reaches up to 1,200 x 106t.year-1 (Meade et al., 1985) and presents proportional amount of nutrients, dissolved and particulate organic carbon dispersed in the Amazon River plume (Valério et al., 2018).
Araguari River, which is contiguous to Amazon River mouth, is the largest and most important river in Amapá State / Brazil; its area comprises approximately 42,700 km², 600 km in length, its mean annual flow is of approximately 1,000 m³-1, and its original course flew into the Atlantic Ocean until 2012 (Santos et al., 2018). However, nowadays, Araguari River flows into Amazon River mouth, rather than into the Atlantic Ocean, due to the phenomenon known as “capture” by the Amazon River, which involves tidal channels, intense erosion processes and hydrodynamic changes originating on the left bank of Amazon River and on the right bank of Araguari River. (Santos et al., 2018). In addition, approximately 55% of the coastline in the North Channel of the Amazon River is undergoing erosion due to dynamic balance disturbances resulting from changes in basin-drainage processes taking place in these ecosystems (Torres et al., 2018).
On the one hand, the effects of these erosive imbalances stand out (Li et al., 2017; Torres et al., 2018), since they favor the emergence of deltas (Santos et al., 2018), as well as increased susceptibility to saline intrusions (Rosário et al., 2016) and geomorphological changes in river mouths (Kolker et al., 2012; Petts and Gurnell, 2005; Silva et al., 2020). Consequently, they affect flood period amplitudes (Junk, et al., 2014, 1996) and biogeochemistry (Allison et al., 2014). Such impacts are intensified by the combination among physical features of the hydrographic basin, loss of connectivity between waterbodies and increased frequency of extreme events associated with climate change (Cunha and Sternberg. 2018).
On the other hand, associated anthropic factors, such as land use and occupation, also lead to sediment retention and transport. The implementation of hydroelectric power plants, extensive non-controlled livestock and navigation systems (Syvitski et al., 2005; Latrubesse et al., 2017; Santos et al., 2018; Silva et al., 2020) stand out among these factors. It is so true that hydrosedimentary imbalances have been causing silting-up of riverbed and banks, as well as impairing the navigability (Bernini et al. 2016) and maintenance of ecosystem services in large river systems presenting significant sedimentary input, such as the Amazon River (Ward et al., 2013).
Despite the ecological-environmental importance of the Amazon River for Brazilian coastal management processes, studies focused on investigating water-sediment interactions and sediment transport processes taking place in it remain scarce in the literature (Torres et al., 2018). Therefore, there are considerable gaps in the literature about the way local, seasonal and tidal factors affect tidal channels and connections between both hydrographic basins. Although Araguari River has collapsed due to the connection and significant expansion of Urucurituba Channel (Santos et al. 2018), it is possible seeing its previous secondary contribution to an upstream channel network, namely: the Gurijuba-Igarapé Novo Channel. These channels appear to compromise the hydrodynamics of Araguari River mouth, in addition to affect the quality of the water, cause temporary salinization, change the hydrological pulse pattern and increase the connectivity between rivers and lakes. Such factors have led to the extinction of pororoca (Cunha and Sternberg 2018) (Figure 1).
The aim of the current study was to elaborate water and seasonal solid load balances to test and correlate hydrogeormorphological changes in the quality of the water in Araguari River, whose mouth collapsed due to “capture” of its flow by the Amazon River, through tidal channels such as Urucurituba and Gurijuba-Igarapé Novo. The study hypothesis is that these two channels got developed through fast erosive processes that started in the Amazon River and propagated towards Araguari River (Santos et al., 2018) (Figure 1).