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. 2018 Apr 16;8:5998. doi: 10.1038/s41598-018-24454-4

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© The Author(s) 2018

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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Comparison of the V-shaped structures on weirs in Bolivia (World Imagery ArcGIS layer, ESRI, Digital Globe, June 2008) and Zambia (satellite image by the Pléiades sensor, July 2013. Pléiades © CNES 2013, distribution Airbus DS). This figure is not covered by the CC BY licence. All rights reserved, used with permission.