Citation by Jim O’Connor
I am pleased to convey the citation for the 2024 Kirk Bryan Award of the Quaternary Geology and Geomorphology Division. Named for the renowned Harvard geomorphologist, it is the flagship award of the QG&G division, and since 1958, it has honored the author or authors of a published paper of distinction advancing the science of geomorphology or some related field award. This year’s Kirk Bryan Award goes to Evan N. Dethier, Carl F. Renshaw, and Francis J. Magilligan for their paper titled “Rapid changes to global river suspended sediment flux by humans” which appeared in the journal Science in June of 2022.
This short paper, supported by extensive supplemental material and a predecessor paper in 2020 by the same authors indeed advances the field of geomorphology: It derives spatially and temporally explicit assessments of suspended sediment flux for large rivers across the globe. The conclusions are profound, in particular clarifying the role of dam building and landuse on global sediment flux, and how those factors have varied temporally and spatially—dam building in the northern hemisphere has reduced sediment flux to the oceans by about half, whereas intensive deforestation, agriculture, and sediment mining of various types have increased southern hemisphere sediment flux by 40 percent. Notably, at broad scales, the effects of climate change are not yet detectable, indicating that current trends of sediment delivery are still overwhelmed by the more direct effects of human activities. But importantly, the approach developed in this paper will enable continued systematic evaluation of sediment transport conditions, thereby facilitating management and evaluation of global sediment conditions.
In principle the premise is simple: the color of a river correlates to its sediment concentration. Anyone that drinks water knows this. But the analysis is not simple, involving algorithms for satellite remote-sensing developed in the 2020 paper and applied to Landsat imagery, for which river spectral signatures are calibrated by a dataset of >130,000 in-situ measurements of suspended sediment concentration. The study applies sophisticated statistical approaches to group rivers efficiently—because different rivers have different ‘color ramps’—and to minimize and quantify uncertainties. In the end, the authors estimated monthly sediment fluxes (and uncertainties) for 414 rivers across the globe for 37 years of satellite observations. Many of these rivers have no other quantitative sediment information. It is these data that enabled further sophisticated analyses of the effects of land-use, dam-building, and climate on sediment fluxes to the oceans, and documents the radical disparity between the northern and southern hemispheres regarding current sediment conditions. All of this work is efficiently distilled into the short and well-crafted Report in Science yet rigorously backed by the accompanying supplemental information and previous work.
This paper and the underlying analysis, relying on ‘big-data’ and ‘machine-learning,’ undoubtedly heralds the future for understanding landscapes. Of course some of us old-timers see such efforts and worry about the place for ‘boots-on-the-ground’ work. I do, perhaps mostly because I could never come close to creating the analyses reported here! But this wonderfully creative and rigorous paper mitigates my angst, showing the immense value and insight that these new approaches can create. In my opinion, this cutting edge and, importantly, socially relevant contribution is exactly the type of paper justifying the Kirk Bryan Award of the Quaternary Geology and Geomorphology Division of the Geological Society of America. Congratulations to Evan, Carl, and Frank for this work.