the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 03 Feb 2026
A new sediment budget for the Congo River Basin reveals underestimated tributary contributions and large-scale deposition
Abstract. Sediment yields from fluvial networks to the global oceans impact land carbon and nutrient cycles and are susceptible to climate, population, and vegetation changes. The Congo River Basin is a frontier for population and land use change, but sediment yield dynamics are poorly constrained within its basin, in particular within its largest tributary, the Kasaï River. To address this, we aimed to (1) introduce a spatially flexible methodology for estimating sediment yield from remote sensing monitoring, (2) establish a budget for the Congo and Kasaï river basins from both major and secondary tributaries, and (3) better constrain depositional losses of sediment during transit in the mainstem. A random forest model was calibrated on Landsat-8 spectral data, and total suspended sediment was accurately predicted (measured vs. predicted R2 = 0.79), though predictions degraded in highly turbid waters due to spectral saturation. A sediment budget of the Congo River revealed that 33.0 Tg yr−1 are exported to the coastal ocean. Most sediment is derived from Congo River headwaters, the Kasaï River, the Oubangui River, and the Aruwimi River, whereas lower-order tributaries contributed 10 % of all sediment inputs. Meanwhile, major contributors to the Kasaï budget (export = 11.1 Tg yr−1) were the Kasaï headwaters, Sankuru, and Kwango-Kwilu rivers. Finally, we found the Cuvette Centrale, a peatland-dominated depression, to be a depositional hotspot, and we estimated its net sediment deposition to be between 5.96 Tg yr−1 and 9.4 Tg yr−1. By monitoring Congo River Basin sediment transport, we aim to provide a better understanding of the Earth-surface processes occurring in a globally significant and rapidly changing watershed that lacks crucial baseline information on its sediment and carbon cycles.
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- RC1: 'Comment on egusphere-2026-247', Anonymous Referee #1, 17 Apr 2026 reply
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- 1
Title: A new sediment budget for the Congo River Basin reveals underestimated tributary contributions and large-scale deposition
overall evaluation
The paper presents a compelling and timely exploration of a crucial Earth-surface system. Its primary strengths lie in its basin-scale scope, the integration of remote sensing with a “sediment-budget” framework, and the provision of new quantitative insights into tributary contributions and net deposition in the Cuvette Centrale. It is relevant to the Journal scope and has the potential to make a significant contribution.
Overall, my assessment is positive, but I believe the manuscript requires more refinement before it is ready for publication. The central conclusions are plausible and generally well-supported, but there are methodological and interpretive issues that need to be elaborated on and presented more clearly. Specifically, the authors should show how uncertainty is propagated, the effects of attrition in calibration data, the methods used for spatial and temporal validation and support the inferred deposition with some evidence. I am also apprehensive about the use of the term “sediment budget,” given that the study quantifies washload-dominated suspended fluxes.
Review against the detailed ESurf criteria
S/N
CRITERIA
1
Does the paper address relevant scientific questions within the scope of ESurf?
Yes, the paper falls within the journal's scope. It examines the sediment budget of the world's second largest river basin and directly contributes to ongoing research in geomorphology, sediment connectivity, and source-to-sink dynamics.
2
Does the paper present novel concepts, ideas, tools, or data?
Yes, it does:
However, the methodological novelty is more applied than conceptual, and the authors should avoid overstating the approach's broad transferability without clearer validation across diverse hydro-sedimentary conditions.
3
Are substantial conclusions reached?
1. The Congo exports approximately 33 Tg/year. This is likely the minimum exported given the study’s focus on washload-dominated suspended fluxes.
2. The Kasaï River contributes more to these exports than previously estimated.
3. Secondary tributaries play a notable role and should not be overlooked.
4. The Cuvette Centrale is likely a major zone for net deposition. Again, this is likely true, but it's important to approach it with caution, as the paper addresses net depositional losses rather than gross deposition, and the supporting evidence is based on inference rather than direct observation.
Are the scientific methods and assumptions valid and clearly outlined?
Partly. The general framework is valid, but some assumptions need more justification
5
Are the results sufficient to support the interpretations and conclusions?
Mostly, but only after more careful qualification
The sediment yield estimates are derived from satellite-based TSS predictions combined with discharge and temporally integrated; however, the methodology is inherently limited in its ability to capture event-scale sediment dynamics. Given the reliance on Landsat imagery, persistent cloud cover, and the use of a ±11-day matching window, high-flow, high-sediment events, likely responsible for a substantial proportion of annual sediment transport, are systematically undersampled or excluded. As a result, the derived sediment yields may be biased toward lower values, reflecting temporally averaged conditions rather than true flux magnitudes. This limitation should be more explicitly acknowledged, and its implications for both total yield and inferred deposition should be discussed
Further, the precision of the quantified depositional estimates and the confidence with which along-river decreases can be attributed specifically to deposition rather than to compounded model error, changes in bias along the mainstem, floodplain exchange, or unresolved tributary/bank inputs is unclear. The paper acknowledges some of this, but the discussion should be strengthened.
6
Is the description of experiments and calculations sufficiently complete and precise to allow reproduction?
Not yet fully, conceptually, yes, but they could include:
7
Do the authors give proper credit to related work and clearly indicate their own contribution?
Yes
8
Does the title clearly reflect the contents of the paper?
The title broadly reflects the scope and findings of the manuscript; however, the use of the term ‘sediment budget’ may overstate the completeness of the analysis. Given the methodological limitations, more qualified wording would improve alignment between the title and the underlying analysis, e.g., “Satellite-derived suspended sediment fluxes in the Congo River Basin indicate underestimated tributary contributions and net downstream sediment losses.”
9
Does the abstract provide a concise and complete summary?
Mostly yes. A minor concern is that it reads slightly more certain than the body of the paper justifies, especially regarding model accuracy and depositional interpretation
10
Is the overall presentation well-structured and clear?
Yes.
11
Is the language fluent and precise?
Yes
12
Are mathematical formulae, symbols, abbreviations, and units correctly defined and used?
Mostly yes.
13
Should any parts of the paper be clarified, reduced, combined, or eliminated?
14
Are the number and quality of references appropriate?
Yes. The references appear appropriate and sufficient.
15
Are the amount and quality of supplementary material appropriate?
They are ok.