Large hydrological systems aggregate compositionally different waters derived from a variety of pathways. In the case of continental‐scale rivers, such aggregation occurs noticeably at confluences between tributaries. Here we explore how such aggregation can affect solute concentration‐discharge (C‐Q) relationships and thus obscure the message carried by these relationships in terms of weathering properties of the Critical Zone. We build up a simple model for tributary mixing to predict the behavior ofC‐Qrelationships during aggregation. We test a set of predictions made in the context of the largest world's river, the Amazon. In particular, we predict that theC‐Qrelationships of the rivers draining heterogeneous catchments should be the most “dilutional” and should display the widest hysteresis loops. To check these predictions, we compute 10 day‐periodicity time series ofQand major solute (Si, Ca²⁺, Mg²⁺, K⁺, Na⁺, Cl^‐,)Cand fluxes (F) for 13 gauging stations located throughout the Amazon basin. In agreement with the model predictions,C‐Qrelationships of most solutes shift from a fairly “chemostatic” behavior (nearly constantC) at the Andean mountain front and in pure lowland areas, to more “dilutional” patterns (negativeC‐Qrelationship) toward the system mouth. More prominentC‐Qhysteresis loops are also observed at the most downstream stations. Altogether, this study suggests that mixing of water and solutes between different flowpaths exerts a strong control onC‐Qrelationships of large‐scale hydrological systems.

In 2023, an unprecedented drought and heat wave severely affected Amazon waters, leading to high mortality of fishes and river dolphins. Five of 10 lakes monitored had exceptionally high daytime water temperatures (over 37°C), with one large lake reaching up to 41°C in the entire approximately 2-meter-deep water column and up to 13°C of diel variation. Modeling showed that high solar radiation, reduced water depth and wind speed, and turbid waters were the main drivers of the high temperatures. This extreme heating of Amazon waters follows a long-term increase of 0.6°C/decade revealed by satellite estimates across the region’s lakes between 1990 and 2023. With ongoing climate change, temperatures that approach or exceed thermal tolerances for aquatic life are likely to become more common in tropical aquatic systems.

The Tropical Atlantic is facing a massive proliferation of Sargassum since 2011, with severe environmental and socioeconomic impacts. As a contribution to this proliferation, an increase in nutrient inputs from the tropical rivers, in response to climate and land use changes or increasing urbanization, has been often suggested and widely reported in the scientific and public literature. Here we discuss whether changes in river nutrient inputs could contribute to Sargassum proliferation in the recent years or drive its seasonal cycle. Using long-term in situ and satellite measurements of discharge, dissolved and particulate nutrients of the three world largest rivers (Amazon, Orinoco, Congo), we do not find clear evidences that nutrient fluxes may have massively increased over the last 15 years. Moreover, focusing on year 2017, we estimate that along the year only 10% of the Sargassum biomass occurred in regions under river plume influence. While deforestation and pollution are a reality of great concern, our results corroborate recent findings that hydrological changes are not the first order drivers of Sargassum proliferation. Besides, satellite observations suggest that the major Atlantic river plumes suffered a decrease of phytoplankton biomass in the last two decades. Reconciling these observations requires a better understanding of the nutrient sources that sustain Sargassum and phytoplankton growth in the region.

This article analyzes the flows of water and total suspended sediment in different reaches in the lower course of the Negro River, the largest fluvial blackwater system in the world. The area under study is the Anavilhanas Archipelago, which is a complex multichannel reach on the Negro River. Between the years 2016 and 2019, data about water discharge, velocity, and concentration of total suspended solids (TSS) were acquired in sample sections of the Negro River channels located upstream, inside, and downstream of the Anavilhanas Archipelago. In the study area, the Negro River drains an area greater than 700,000 km², and the mean water discharge observed before the Anavilhanas was about 28.655 m³·s⁻¹, of which 97% flows through two channels of the Archipelago close to the right and left banks. The mean TSS concentration of the Negro River upstream and downstream the Archipelago was 3.28 mg·L⁻¹ and 1.63 mg·L⁻¹, respectively. Within the Archipelago, we observed more TSS in the channel on the left bank of the Negro River (mean of 4.50 mg·L⁻¹). The total suspended sediment discharge of the Negro River before (3.14 Mt·year⁻¹) and after (1.43 Mt·year⁻¹) the Anavilhanas Archipelago indicates a 55% retention of the suspended load due to the low water slope and reduced flow velocity caused by the backwater effect of Solimões River on the Negro River. The hydro-sedimentary scenario of the low course of the Negro River characterized in this study indicates a slow and continuous sedimentation process in the Anavilhanas Archipelago. The results presented will serve as a baseline to assess the impacts of the dams on the Branco River, the main tributary for both water and sediment in the Negro River basin.