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).