Rivers bigger and more important that previously thought 2018-06-28

Rivers and streams cover more of the earth’s surface than previously thought, and likely interchange more CO2 and other gases with the atmosphere than previously thought. WWALS Science Committee Chair Tom Potter found this paper.

George H. Allen and Tamlin M. Pavelsky, Science, 28 Jun 2018, DOI: 10.1126/science.aat0636 Global extent of rivers and streams,

Abstract

The turbulent surfaces of rivers and streams are natural hotspots of biogeochemical exchange with the atmosphere. At the global scale, the total river-atmosphere flux of trace gasses such as CO2 depends on the proportion of Earth’s surface that is covered by the fluvial network, yet the total surface area of rivers and streams is poorly constrained. We used a global database of planform river hydromorphology and a statistical approach to show that global river and stream surface area at mean annual discharge is 773,000 ± 79,000 km2 (0.58 ± 0.06%) of Earth’s non-glaciated land surface, an area 44 ± 15% larger than previous spatial estimates. We found that rivers and streams likely play a greater role in controlling land-atmosphere fluxes than currently represented in global carbon budgets.

Fig. 1. Global River Widths from Landsat (GRWL) Database, Figure
Fig. 1. The Global River Widths from Landsat (GRWL) Database contains more than 58 million measurements of planform river geometry. The line plot on the right shows observed river coverage as a percentage of land area by latitude, and the bottom insets show GRWL at increasing zoom. The rightmost inset shows GRWL orthogonals over which river width was calculated, with only every eighth orthogonal shown for clarity.

You can see the lower Suwannee River in the above figure.

The authors zoom in on the Amazon River Basin in Brazil, but those last two zooms could easily be the area of the Alapaha River near the Willacoochee Confluence that we just paddled. Our rivers are smaller, sure, but the patterns are very similar, including hairpin turns, oxbows starting to be cut across their necks by the river, forming eyots: islands in the stream.

The authors mention “lakes, reservoirs, and canals connected to the fluvial network” but not swamps or seasonal floodplains. So the effects of our swampy often-flooded rivers could be more than they estimate.

Towards the end of the paper there are interesting mentions of human intervention:

Compared with the current best region-by-region global estimate (1), we found more river and stream coverage in the Arctic and less in Europe, the conterminous United States, and some other economically developed regions (Fig. 4C). Previous estimates of global RSSA do not consider extra-climatic influences on RSSA, such as variations in fluvial geomorphology and human modifications to river channels, potentially resulting in an overestimate in some developed regions. For example, RSSA in many developed regions may be less than pre- viously predicted owing to the influence of leveeing and water withdrawal in these areas.

RSSA is river and stream surface area. We are familiar with water withdrawal in south Georgia and north Florida, for agriculture and industry, and especially development sucking up water.

The authors mention interchange with the atmosphere works both ways: the air affects the termperature and chemistry of river and stream water. For an example they don’t use, mercury in the Alapaha River comes through the air from the country’s dirtiest coal plant, Plant Scherer north of Macon, Georgia, not even in the Suwannee River Basin.

The downward revision of RSSA in economically developed regions may be related to the large-scale impact of human modification on the fluvial network, although this hypothesis requires further testing. The largest sources of unquantified uncertainty in our RSSA estimate likely originate from the distribution of surface area for intermediate-sized rivers and streams and the seasonal variation of RSSA within river networks.

They say they’re working on those things. That last sentence may get into floodplains. We shall see what else they publish after this very recent and very interesting paper.

 -jsq, John S. Quarterman, Suwannee RIVERKEEPER®

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