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We analyzed two contrasting catchments located among the world's largest unspoiled tropical rainforests impacted by mining in the northeastern coastlands of South America. We used the following: (a) mining, agricultural, and urbanized areas to compare the land use evolution with suspended sediments and sediment yields; (b) field monthly river suspended sediments in the two catchments (2004–2015: n = 154); (c) MODIS remote sensing water color technique in the Maroni basin to complete (n = 387) and extend field suspended sediment sampling from 2000 to 2015; (d) hydroclimatic statistical analysis conditions and sediment concentrations to identify the long‐term trends, the abrupt changes in time series and to analyze if the environmental and anthropogenic factors control sediment yield regional variations. No significant long‐term changes were observed in precipitation or water discharge with the Mann–Kendall test. However, the mean suspended sediment concentration has increased significantly (239%) in the Maroni River with a breakpoint in 2009 and decreased (33%) in the Oyapock River (breakpoint in 2008). These differences are explained by the larger percentage of deforestation because of mining activities in the Maroni (0.37%) than in the Oyapock (0.06%) catchment. In the Maroni River, the increasing sediment yield trend (2000–2015) coincide significantly (r2 = 0.97; p < 0.0001) with the increase of 400% of mining areas, whereas no significant relationship with the runoff was found. In the Oyapock River, the runoff explains the sediment yield decreasing trend (r2 = 0.82; p < 0.0001) and no relationship with the land use change was found.

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Deforestation and mining activities have proven to be very damaging to rivers because these activities disturb the environmental characteristics of rivers. Thus, the concentrations of dissolved organic carbon (DOC), particulate organic carbon (POC), particulate nitrogen (PN), and Chlorophyll-a (Chl-a) were measured monthly during 2 hydrological years in the Maroni and Oyapock Rivers to assess the dynamics and fluxes of organic carbon and nitrogen in these 2 Guiana Shield basins, which have been strongly (Maroni) and weakly (Oyapock) impacted by deforestation and mining activities. The 2-year time series show that DOC, POC, PN, and Chl-a concentrations vary seasonally with discharge in both rivers, indicating a hydrologically dominated control. Temporal patterns of DOC, POC, and PN indicate that these variables show maximum concentrations in rising waters due to the yield of organic matter and nitrogen accumulated in soils, which are incorporated into the rivers during rainfall. However, the Chl-a concentrations were at a maximum during low-water stages. The C/N and C/Chl-a ratios also showed a seasonal trend, with lower values during the low water periods due to an increase in algal biomass. During high water, the POC in both rivers is the result of terrestrial organic matter, whereas during low-water autochthonous organic matter can reach up to 34% of the POC. The mean annual fluxes of TOC and PN were higher (4.56 × 10 5 tonC year −1 and 1.77 × 10 4 tonN year −1 , respectively) in the Maroni River than those (1.84 × 10 5 tonC year −1 and 0.54 × 10 4 tonN year −1 , respectively) in the Oyapock River. However, the specific fluxes of DOC, POC, and PN from both basins were nearly the same. Although gold mining activities are performed in both basins, there is no conclusive evidence regarding the impact of these activities on the dynamics of organic matter and particulate nitrogen in the Maroni and Oyapock Rivers.

&lt;p&gt;In a context of a changing climate and an increasing anthropic pressure on natural resources, it is more than ever necessary to maintain or even improve our capacity to understand and monitor inland waters. It is particularly the case in French Guyana, where, despite its relative density, the in situ monitoring network fails at providing everyday information on rivers all over the territory. The use of free and open spatial datasets and hydrological modeling have shown great skills in complementing existing monitoring networks all over the world.&amp;#160;&amp;#160;&lt;/p&gt; &lt;p&gt;Our work illustrates the use of a hydrological model, namely the MGB, set-up for 10 major watersheds in French Guiana (including the transboundary Maroni and Oyapock River - resp. with Suriname and Brazil- and some smaller ungauged basins) fed on a daily and near-real-time basis by IMERG-RT (Integrated Multi-satellitE Retrievals for GPM - Real Time) remote sensing precipitation products within a scheduler (namely HYFAA). In ungauged basins we used model parameters regionalisation to infer model parameters. The model performed well at inferring discharges, with KGE values higher than 0.7 when compared to gages. An extended dataset of rating curve between water surface elevation from nadir altimetry and simulated discharges is extracted using a physical-based processing of radar echoes on ESA Sentinel3 A&amp;B and Jason3/Sentinel6 missions and also the time series available on Hydroweb website (https://hydroweb.theia-land.fr/). The quality of the rating curves confirms the skill of the model even in ungauged locations and watersheds with small contributive area.&amp;#160;&lt;/p&gt; &lt;p&gt;Thanks to this set-up, discharges and water levels are estimated daily all over the territory, and routinely corrected by the use of satellite altimetry. Using statistical rainfall predictions and watersheds concentration time, the system allows short-term forecasts of the discharge. In coordination with the &lt;em&gt;in situ&lt;/em&gt; network operator, the critical thresholds were defined and are used to trigger&amp;#160; flood and droughts alerts, accessible online and received by email upon registration. As the methods used in this study have largely proved to be deployable anywhere, this simple framework draws the contour of future operational early warning systems based on space observation.&amp;#160;&lt;/p&gt;