48. Wesselink, A.; Orange, D.; Feizouré, C.; Randriamiarisoa. Les régimes hydroclimatiques et hydrologiques d'un bassin versant de type tropical humide: L'Oubangui (République Centrafricaine). In *L'hydrologie Tropicale: Géosciences et Outil Pour le Développement: Mélanges à la Mémoire de Jean Rodier*; Chevallier, P., Pouyaud, B., Eds.; IAHS: Wallingford, UK, 1996; Volume 238, pp. 179-194.

49. Olivry, J.-C.; Bricquet, J.-P.; Mahé, G. Vers un appauvrissement durable des ressources en eau de l'Afrique humide. In *Hydrology of Warm Humid Regions*. (Proc. Yokohama Symp., July 1993); Gladwell, J.S., Ed.; IAHS: Paris, France, 1993; Volume 216, pp. 67-78.

50. Mahé, G.; Olivry, J.-C. Variations des précipitations et des écoulements en Afrique de l'Ouest et centrale de 1951-1989. *Sci. et Chang. planétaires/Sécheresse* **1995**, *6*, 109-117.

51. Bricquet, J.-P. Les écoulements du Congo à Brazzaville et la spatialisation des apports. In *Grands Bassins Fluviaux Périatlantiques: Congo, Niger, Amazone-Actes du colloque PEGI-INSU-CNRS-ORSTOM*; Olivry, J.-C., Boulègue, J., Eds.; ORSTOM: Paris, France, 1995; pp. 27-38.

52. Bricquet, J.-P.; Bamba, F.; Mahe, G.; Toure, M.; Olivry, J.C. Evolution récente des ressources en eau de l'Afrique atlantique. *Rev. des Sci. de l'Eau* **1997**, *3*, 321-337.

53. Lee, H.; Beighley, R.E.; Alsdorf, D.; Jung, H.C.; Shum, C.K.; Duan, J.; Guo, J.; Yamazaki, D.; Andreadis, K. Characterization of terrestrial water dynamics in the Congo Basin using GRACE and satellite radar altimetry. *Remote Sens. Environ.* **2011**, *115*, 3530-3538. [CrossRef]

54. Lee, H.; Jung, H.-C.; Yuan, T.; Beighley, R.E.; Duan, J. Controls of terrestrial water storage changes over the central Congo Basin determine by integrating PalSAR scanSAR, EnviSat altimetry, and GRACE data. In *Proceedings of the Chapman Conference, Washington, DC, USA, 25 September 2018; Volume 2014*.

55. Becker, M.; Santos da Silva, J.; Calmant, S.; Robinet, V.; Linguet, L.; Seyler, F. Water Level Fluctuations in the Congo Basin Derived from ENVISAT Satellite Altimetry. *Remote Sens. 2014*, *6*, 9340-9358. [CrossRef]

56. Moukandi N'kaya, G.D.; Orange, D.; Bayonne Padou, S.; Datok, P.; Laraque, A. Temporal Variability of Sediments, Dissolved Solids and Dissolved Organic Matter Fluxes in the Congo River at Brazzaville/Kinshasa. *Geosciences* **2020**, *10*, 341. [CrossRef]

57. Mahé, G.; Lerique, J.; Olivry, J.-C. L'Ogooué au Gabon. Reconstitution des débits manquants et mise en évidence de variations climatiques à l'équateur. *Hydrol. Continent. 1990*, *5*, 105-124.

58. Liénou, G.; Mahé, G.; Paturel, J.E.; Servat, E.; Sighomnou, D.; Sigha-Nkamdjou, L.; Ekodeck, G.E.; Dezetter, A.; Dieulin, C. Impact de la variabilité climatique en zone équatoriale: Exemple de modification de cycle hydrologique des rivières du sud-Cameroun. *Hydrol. Sci. J. Sci. Hydrol.* **2008**, *53*, 789-801. [CrossRef]

59. Hubert, P. The segmentation procedure as a tool for discrete modeling of hydro-meteorological regimes. *Stoch. Environ. Res. Risk Assess.* **2000**, *14*, 297-304. [CrossRef]

60. Moron, V.; Bigot, S.; Roucou, P. Rainfall variability in subequatorial America and Africa and relationships with the main sea-surface temperatures modes (1851-1990). *Int. J. Clim.* **1995**, *15*, 1297-1322. [CrossRef]

61. Tshitenge Mbuebue, J.-M.; Lukanda Mwamba, V.; Tshimanga Muamba, R.; Javaux, M.; Mahe, G. Wavelet Analysis on the Variability and the Teleconnectivity of the Rainfall of the Congo Basin for 1940-1999. In *Proceedings of the Conférence Internationale sur l'hydrologie des grands bassins Fluviaux d'Afrique, Hammamet, Tunisia, 26-30 October 2015*.

62. Lubes Niel, H.; Masson, J.M.; Paturel, J.E.; Servat, E. Variabilité climatique et statistiques. Etude par simulation de la puissance et de la robustesse de quelques tests utilisés pour vérifier l'homogénéité de chroniques. *Rev. des Sci. de l'Eau* **1998**, *11*, 383-408. [CrossRef]

63. KHRONOSTAT. *Logiciel d'analyse Statistique de Séries Chronologiques*; IRD Ex: Paris, France; Available online: http://www.hydrosciences.org/spip.php?article239 (accessed on 14 October 1999). (In French)

64. WMO. *World Meteorological Organization, Climatic Change. Report of a Working Group of the Commission for Climatology*; WMO 195, TP 100, Tech. note n°79; WMO: Geneva, Switzerland, 1966.

65. Laraque, A.; Mahé, G.; Orange, D.; Marieu, B. Spatiotemporal Variations in hydrological regimes within Central Africa during the XXth century. *J. Hydrol.* **2001**, *245*, 104-117. [CrossRef]

66. Nguimalet, C.R.; Orange, D. Evolution annuelle de l'indice pluviométrique sur le bassin de l'Oubangui à Mobaye (1938-2015). In *Congo Basin-Hydrology, Climate, and Biogeochemistry: A Foundation for the Future*; Alsdorf, D., Tshimanga, M.R., Moukandi N'kaya, G.D., Eds.; AGU, John Wiley & Sons Inc.: Malden, MA, USA, 2021; Unpublished work.

67. Pham-Duc, B.; Sylvestre, F.; Papa, F.; Frappart, F.; Bouchez, C.; Crétaux, J.-F. The Lake Chad hydrology under current climate change. *Nat. Sci. Rep.* **2020**, *10*, 5498. [CrossRef]
68. Nicholson, S. Climate and climatic variability of rainfall over eastern Africa. *Rev. Geoph.* **2017**. [CrossRef]

69. van Mierlo, J.G. Le mécanisme des alluvions du Congo. *Annls. Ass. Ingrs. Éc Gand.* **1926**, *5*, 349–354.

70. Spronck, R. Mesures hydrographiques dans la région divagante de bief maritime du fleuve Congo. *Mém. Inst. R. Colon. Sect. Sci. Technol.* **1941**, *3*, 56.

71. Eisma, D.; Kalf, J.; Vandergaast, S.J. Suspended Matter in the Zaire Estuary and the Adjacent Atlantic Ocean. *Neth. J. Sea Res.* **1978**, *12*, 382–406. [CrossRef]

72. Coynel, A.; Seyler, P.; Etcheber, H.; Meybeck, M.; Orange, D. Spatial and seasonal dynamics of total suspended sediment and organic carbon species in the Congo River. *Glob. Biogeochem. Cycles* **2005**, *19*, GB4019. [CrossRef]

73. Spencer, R.G.M.; Hernes, P.J.; Aufdenkampe, A.K.; Baker, A.; Gulliver, P.; Stubbins, A.; Aiken, G.R.; Dyda, R.Y.; Butler, K.D.; Mwamba, V.L.; et al. An initial investigation into the organic matter biogeochemistry of the Congo River. *Geochim. Cosmochim. Acta* **2012**, *84*, 614–627. [CrossRef]

74. Spencer, R.G.M.; Hernes, P.J.; Dinga, B.; Wabakanghanzi, J.N.; Drake, T.W.; Six, J. Origins, seasonality, and fluxes of organic matter in the Congo River. *Glob. Biogeochem. Cycles* **2016**, *30*. [CrossRef]

75. Seyler, P.; Coynel, A.; Moreira-Turcq, P.; Etcheber, H.; Colas, C.; Orange, D.; Bricquet, J.-P.; Laraque, A.; Guyot, J.L.; Meybeck, M. Organic carbon transported by the equatorial rivers: Example of Zaire-Congo and Amazon Rivers. In *Soil Erosion and Carbon Dynamics*; Roosevelt, E.J., Lal, R., Feller, C., Barthes, B., Stewart, B.A., Eds.; CRC Press: Boca Raton, FL, USA, 2006; Volume 15, pp. 255–274.

76. Bouillon, S.; Yambélé, A.; Gillikin, D.P.; Teodoru, C.; Darchambeau, F.; Lambert, T.; Borges, A.V. Contrasting biogeochemical characteristics of the Oubangui River and tributaries (Congo River basin). *Sci. Rep.* **2014**, *4*, 5402. [CrossRef]

77. Devroey, E. *Le Bassin Hydrographique Congolais*; Coll Mémoires in-8° de la Section des Sciences Techniques Institut Royal Colonial Belge: Bruxelles, Belgium, 1941; p. 172.

78. Nedeco (Netherlands Engineering Company). *River Studies, Niger and Benué*; North Holland Publishing Co.: Amsterdam, The Netherlands, 1959.

79. Gibbs, R.J. Amazon river-environmental factors that control its dissolved and suspended load. *Science* **1967**, *156*, 1203–1232. [CrossRef]

80. Symoens, J.J. La minéralisation des eaux naturelles. Résultats scientifiques. *Explor. Hydrobiol. Bassin du Lac Bangwelo et du Luapula* **1968**, *2*, 1–199.

81. Holleman, J.N. The sediment yield of major rivers of the world. *Water Resour. Res.* **1968**, *4*, 737–747. [CrossRef]

82. Vanderlinden, M.J.H. Reactions between Acids and Leaf Literi; Premier colloque Int. Biodegradation et humification. Ph.D. Thesis, Univ. Nancy, Nancy, France, 1975.

83. Meybeck, M. Note on elementals contents of the Zaire River. *Neth. J. Sea Res.* **1978**, *12*, 293–295. [CrossRef]

84. Meybeck, M. Concentrations des eaux fluviales en éléments majeurs et apports en solution aux océans. *Revue de Géologie Dyn. Géogr. Phys.* **1979**, *21*, 215–246.

85. Molinier, M. Note sur les débits et la qualité des eaux du Congo à Brazzaville. *Cah. ORSTOM Série Hydrol.* **1979**, *16*, 55–66.

86. Deronde, L.; Symoens, J.J. L'exportation des éléments dominants du bassin du fleuve Zaïre: Une réévaluation. *Ann. Limnol.* **1980**, *16*, 183–188. [CrossRef]

87. Grondin, J.L.; Gac, J.Y. Apports de matières aux océans: Bilan des six principaux fleuves d'Afrique. 1980; unpublished work.

88. Eisma, D. Suspended matter as a carrier for pollutants in estuaries and the sea. In *Marine Environmental Pollution, 2. Mining and Dumping*; Geyer, R.A., Ed.; Elsevier: Amsterdam, The Netherlands, 1982; Volume 27, pp. 281–295.

89. Meybeck, M. Carbon, nitrogen and phosphorus transport by world rivers. *Am. J. Sci.* **1982**, *282*, 401–450. [CrossRef]

90. Kinga-Mouzeo. Transport Particulaire Actuel du Fleuve Congo et de Quelques Affluents; Enregistrement Quaternaire dans L'éventail Détrique Profond (Sédimentologie, Minéralogie et Géochimie). Ph.D. Thesis, Université de Perpignan, Perpignan, France, 1986; p. 251.

91. Nkounkou, R.; Probst, J.-L. Hydrology and geochemistry of the Congo River system. *SCOPE/UNEP Sonderband* **1987**, *64*, 483–508.
92. Olivry, J.-C.; Bricquet, J.-P.; Thiébaux, J.-P.; Sigha-Nkamdjou, L. Transport de matière sur les grands fleuves des régions intertropicales: Les premiers résultats des mesures de flux particulaires sur le bassin du fleuve Congo. In *Sediment Budgets*; AISH 174: Porto-Alegre, Brazil, 1988; pp. 509–521.

93. Probst, J.-L.; Nkounkou, R.; Krempp, G.; Bricquet, J.-P.; Thiebaux, J.-P.; Olivry, J.-C. Dissolved major elements exported by the Congo and the Ubangi rivers during the period 1987–1989. *J. Hydrol.* **1992**, *135*, 237–257.
[CrossRef]

94. Moukolo, N.; Laraque, A.; Olivry, J.-C.; Bricquet, J.-P. Transport en solution et en suspension par le fleuve Congo (Zaïre) et ses principaux affluents de la rive droite. *Hydrol. Sci. J.* **1993**, *38*, 133–145. [CrossRef]

95. Olivry, J.-C.; Bricquet, J.-P.; Laraque, A.; Guyot, J.-L.; Bourges, J.; Roche, M.A. Flux liquides, dissous et particulaires de deux grands bassins intertropicaux: Le Congo à Brazzaville et le Rio Madeira à Villabella. In *Grands Bassins Fluviaux Périatlantiques: Congo, Niger, Amazone-Actes du Colloque PEGI-INSU-CNRS-ORSTOM*; Olivry, J.-C., Boulégue, J., Eds.; ORSTOM: Paris, France, 1995; pp. 345–355.

96. Sondag, F.; Laraque, A.; Riandey, C. Chimie des eaux du fleuve Congo à Brazzaville et de l'Oubangi à Bangui (années 1988 à 1992). In *Grands Bassins Fluviaux Périatlantiques: Congo, Niger, Amazone-Actes du colloque PEGI-INSU-CNRS-ORSTOM*; Olivry, J.-C., Boulégue, J., Eds.; ORSTOM: Paris, France, 1995; pp. 121–131.

97. Laraque, A.; Castellanos, B.; Steiger, J.; Lopez, J.-L.; Pandi, A.; Rodriguez, M.; Rosales, J.; Adèle, G.; Perez, J.; Lagane, C. A comparison of the suspended and dissolved matter dynamics of two large inter-tropical rivers draining into the Atlantic Ocean: The Congo and the Orinoco. *Hydrol. Process.* **2013**. [CrossRef]

98. Borges, A.V.; Darchambeau, F.; Lambert, T.; Morana, C.; Allen, G.H.; Tambwe, E.; Toengaho Sembaito, A.; Mambo, T.; Nlandu Wabakhangazi, J.; Descy, J.-P.; et al. Variations in dissolved greenhouse gases (CO₂, CH₄, N₂O) in the Congo River network overwhelmingly driven by fluvial-wetland connectivity. *Biogeoscience* **2019**, *16*, 3801–3834. [CrossRef]

99. Mushi, C.A.; Ndomba, P.M.; Trigg, M.A.; Tshimanga, R.M.; Mtalo, F. Assessment of basin-scale soil erosion within the Congo River Basin: A review. *Catena* **2019**, *178*, 64–76. [CrossRef]

100. Burgis, M.J.; Symoens, J.J. *African Wetlands and Shallow Water Bodies-Zones Humides et lacs peu Profonds d'Afrique*; ORSTOM: Paris, France, 1987; p. 650.

101. O'Loughlin, F.; Trigg, M.A.; Schumann, G.J.-P.; Bates, P.D. Hydraulic characterization of the middle reach of the Congo River. *Water Res. Resear.* **2013**, *49*, 5059–5070. [CrossRef]

102. Betbeder, J.; Gond, V.; Frappart, F.; Baghdadi, N.N.; Briant, G.; Bartholomé, E. Mapping of central Africa forested wetlands using remote sensing. *IEEE J. Sel. Top. Appl. Earth Obs. Remote Sens.* **2013**. [CrossRef]

103. Alsdorf, D.; Beighley, D.; Laraque, A.; Lee, H.; Tshimanga, R.; O'Loughlin, F.; Mahé, G.; Dinga, B.; Moukandi N'kaya, G.; Spencer, R.G.M. Opportunities for hydrologic research in the Congo Basin. *Rev. Geophys.* **2016**, *54*. [CrossRef]

104. Laraque, A.; Pouyaud, B.; Rocchia, R.; Robin, R.; Chaffaut, I.; Moutsambote, J.M.; Maziezoula, B.; Censier, C.; Albouy, Y.; Elenga, H.; et al. Origin and function of a closed depression in equatorial humid zones: The lake Tele in the north Congo. *J. Hydrol.* **1998**, *207*, 236–253. [CrossRef]

105. Marlier, G. Limnology of the Congo and Amazon Rivers. In *Tropical Forest Ecosystems in Africa and South America: A Comparative Review*; Meggers, B.J., Ayensu, E.S., Duckworth, W.D., Eds.; Smithsonian Institution Press: Washington, DC, USA, 1973; pp. 223–238.

106. Alsdorf, D.; Bates, P.; Melack, J.; Wilson, M.; Dunne, T. The spatial and temporal complexity of the Amazon flood measured from space. *Geophys. Res. Lett.* **2007**, *34*, L08402. [CrossRef]

107. Rudorff, C.M.; Melack, J.M.; Bates, P.D. Flooding dynamics on the lower Amazon floodplain: 2 Seasonal and interannual\*hydrological variability. *Water Resour. Res.* **2014**, *50*, 635–649. [CrossRef]

108. Bonnet, M.-P.; Barroux, G.; Seyler, P.; Pecly, G.; Moreira-Turcq, P.; Lagane, C.; Cochoneau, G.; Viers, J.; Seyler, F.; Guyot, J.-L. Seasonal links between the Amazon corridor and its flood plain: The case of the varzea of Curuai. In *Dynamics and Biogeochemistry of River Corridors and Wetlands*. In Proceedings of the Symposium Held During the Seventh IAHS Scientific Assembly, Foz do Iguaçu, Brazil, 3–9 April 2005; p. 294.

109. Martinez, J.M.; Le Toan, T. Mapping of flood dynamics and vegetation spatial distribution in the Amazon floodplain using multitemporal SAR data. *Remote Sens. Environ.* **2007**, *108*, 209–223. [CrossRef]

110. Trigg, M.A.; Bates, P.D.; Wilson, M.D.; Schumann, G.; Baugh, C. Floodplain channel morphology and networks of the middle Amazon River. *Water Resour. Res.* **2012**, *48*. [CrossRef]
111. Tshimanga, R.M. Two decades of hydrologic modeling and predictions in the Congo River Basin: Progress and prospect for future investigations. In *Congo Basin-Hydrology, Climate, and Biogeochemistry: A Foundation for the Future*; Alsdorf, D., Tshimanga Muamba, R., Moukandi N’kaya, G.D., Eds.; AGU, John Wiley & Sons Inc.: Malden, MA, USA, 2021; Unpublished work.

112. Beighley, R.E.; Ray, R.L.; He, Y.; Lee, H.; Schaller, L.; Andreadis, K.M.; Durand, M.; Alsdorf, D.; Shum, C.K. Comparing satellite derived precipitation datasets using the Hillslope River Routing (HRR) model in the Congo River Basin. *Hydrol. Process.* **2011**, *25*, 3216–3229. [CrossRef]

113. Tshimanga, R.M.; Hughes, D.A. Basin scale performance of a semi distributed rainfall runoff model for hydrological predictions and water resources assessment of large rivers: The Congo River. *Water Resour. Res.* **2014**. [CrossRef]

114. Aloysius, N.; Saiers, J. Simulated hydrologic response to projected changes in precipitation. *Hydrol. Earth Syst.* **2017**, *21*, 4115–4130. [CrossRef]

115. Munzimi, Y. Satellite-Derived Rainfall Estimates (TRMM Products) Used for Hydrological Predictions of the Congo River Flow: Overview and Preliminary Results. START Report. 2008. Available online: http://start.org/alumni-spotlight/yolande-munzimi.html (accessed on 12 January 2020).

116. Kabuya, P.M.; Hughes, D.A.; Tshimanga, R.M.; Trigg, M.A. Understanding factors influencing the wetland parameters of a monthly rainfall-runoff model in the Upper Congo River Basin. In Proceedings of the EGU Conference Presentation, EGU2020-642, (in the Format Sharing Geoscience Online). 4–8 May 2020.

117. Datok, P.; Fabre, C.; Sauvage, S.; Moukandi N’kaya, G.D.; Paris, A.; Dos Santos, V.; Laraque, A.; Sánchez Pérez, J.M. Investigating the role of the central cuvette of the Congo River in the hydrology of the basin. In *Congo Basin-Hydrology, Climate, and Biogeochemistry: A Foundation for the Future*; Alsdorf, D., Tshimanga Muamba, R., Moukandi N’kaya, G.D., Eds.; AGU, John Wiley & Sons Inc.: Malden, MA, USA, 2021; Unpublished work.

118. Paris, A.; Calmant, S.; Gosset, M.; Fleischmann, A.; Conchy, T.; Garambois, P.A.; Bricquet, J.-P.; Papa, F.; Tshimanga, R.; Moukandi N’kaya, G.D.; et al. Monitoring hydrological variables from remote sensing and modelling in the Congo River basin. In *Congo Basin-Hydrology, Climate, and Biogeochemistry: A Foundation for the Future*; Alsdorf, D., Tshimanga Muamba, R., Moukandi N’kaya, G.D., Eds.; AGU, John Wiley & Sons Inc.: Malden, MA, USA, 2021; Unpublished work.

119. Carr, A.; Trigg, M.A.; Tshimanga, R.; Smith, M.W.; Borman, D.J.; Bates, P.D. Modelling the Bathymetry and Hydraulics of the Congo River Multichannel Mainstem using Spatially Limited In-situ Data. In *Congo Basin-Hydrology, Climate, and Biogeochemistry: A Foundation for the Future*; Alsdorf, D., Tshimanga Muamba, R., Moukandi N’kaya, G.D., Eds.; AGU, John Wiley & Sons Inc.: Malden, MA, USA, 2021; Unpublished work.

120. Trigg, M.; Carr, A.B.; Smith, M.W.; Tshimanga, R. Measuring Geomorphological Change on the Congo River Using Century Old Navigation Charts. Geomorphological Change on the Congo River. In *Congo Basin-Hydrology, Climate, and Biogeochemistry: A Foundation for the Future*; Alsdorf, D., Tshimanga Muamba, R., Moukandi N’kaya, G.D., Eds.; AGU, John Wiley & Sons Inc.: Malden, MA, USA, 2021; Unpublished work.

121. Molliex, S.; Kettner, A.J.; Laurent, D.; Droz, L.; Marsset, T.; Laraque, A.; Rabineau, M.; Moukandi N’Kaya, G.D. Simulating sediment supply from the Congo watershed over the last 155 ka. *Q. Sci. Rev.* **2019**, *203*, 38–55. [CrossRef]

122. Pandi, A.; Ibiassi, G.; Tondo, B.; Ladel, J.; Laraque, A. Impact de la variabilité des écoulements sur la navigabilité de l'Oubangui, un affluent du fleuve Congo. In Proceedings of the International Conference, (WCCCAA: Water issues/challenges and Climate Change Adaptation in Africa), Bangui, Central African Republic, 24–28 October 2012; pp. 38–39.

123. ATC; ACCF; SCEVN. *Renseignements Sur Les Conditions de Navigabilité du Fleuve Congo et Rivières du Bassin Congo-Oubangui-Sangha*; Rapport Technique: Brazzaville, République du Congo, 2000; p. 69.

124. CICOS. *Atlas du Bassin du Congo*; CICOS: San Jose, CA, USA, 2016; p. 95.

125. Wachter, S.J. Giant Dam Projects Aim to Transform African Power Supplies. *The New York Times*. 19 June 2007. Available online: https://www.nytimes.com/2007/06/19/business/worldbusiness/19iht-rnrghydro.1.6204822.html (accessed on 25 February 2020).

126. Umolu, J.C. Macro perspectives for Nigeria's water resources planning. Discussion of Ubangi-Lake Chad diversion schemes. In Proceedings of the First Biennial National Hydrology Symposium, Maiduguri, Nigeria, 26–28 November 1990; pp. 218–262.
127. Kiari Fougou, H.; Lemoalle, J. Variabilité du lac Tchad: Quelle gestion hydraulique pour préserver les ressources naturelles? In *Congo Basin-Hydrology, Climate, and Biogeochemistry: A Foundation for the Future*; Alsdorf, D., Tshimanga Muamba, R., Moukandi N’kaya, G.D., Eds.; AGU, John Wiley & Sons Inc.: Malden, MA, USA, 2021; Unpublished work.

128. Callède, J.; Ronchail, J.; Guyot, J.L.; Oliveira, E. Amazonian deforestation: Its influence on the Amazon discharge at Óbidos (Brasil). *Rev. Sci. L Eau* **2008**, *21*, 59-72.

129. Milly, P.C.D.; Betancourt, J.; Falkenmark, M.; Hirsch, R.M.; Kundzewicz, Z.W.; Lettenmaier, D.P.; Stouffer, R.J. Stationarity is dead: Whither water management? *Science* **2008**, *319*, 573-574. [CrossRef]

130. Milly, P.C.D.; Betancourt, J.; Falkenmark, M.; Hirsch, R.M.; Kundzewicz, Z.W.; Lettenmaier, D.P.; Stouffer, R.J.; Dettinger, M.D.; Krysanova, V. On critiques of ‘Stationarity is Dead: Whither water management?’. *Water Res. Resear.* **2015**, *51*, 7785-7789. [CrossRef]

<figure><img src="image_201.png" /></figure>

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
<figure><img src="image_202.png" /></figure>

Article

# Water and Sediment Budget of Casiquiare Channel Linking Orinoco and Amazon Catchments, Venezuela
†

Alain Laraque<fnref n="1" />, Jose Luis Lopez<fnref n="2" />, Santiago Yepez <fnref n="3" />,* and Paul Georgescu <fnref n="4" />

Received: 4 September 2019; Accepted: 27 September 2019; Published: 3 October 2019

**Abstract:** The Casiquiare River is a natural channel that connects two of the greatest rivers in the world, the Orinoco and the Amazon in the South American continent. The aim of this paper is to present a review and synthesis of the hydrological and sedimentological knowledge of the Casiquiare River, including the first hydro-sedimentary balance of the Casiquiare fluvial system conducted 9–12 September 2000 at the bifurcation and mouth during the expedition 'Humboldt-Amazonia 2000'. Bathymetric flow discharge and physico-chemical measurements were made at the inlet and outlet of the Casiquiare Channel. The main conclusions of this study indicate that Casiquiare is taking a significant proportion of flow (20% to 30%) from the Upper Orinoco basin to the Amazon basin. Throughout its 356 km-course, this chameleon channel undergoes significant morphological, hydrological, and bio-geochemical variations between the inlet and outlet, whose most visible witnesses are the increase in its width (3 to 4 times), flow (7 to 9 times), and its change in water color (white to black water), under the influence of tributaries coming from vast forest plains.

**Keywords:** Casiquiare; Orinoco; Amazon; bifurcation; hydro-sedimentary budget

## 1. Introduction

The Casiquiare River is a geographical rarity and a true hydrological anomaly. It is a natural channel that connects two of the greatest rivers in the world, the Orinoco and the Amazon rivers (Figure 1A). Located on the border between Venezuela and Brazil, the Upper Orinoco deviates part of its waters to the Amazon basin. This is the largest example of river capture in the world. Thus, the same source feeds two different marine estuaries on the Atlantic coast of the South American continent, which makes it unique, taking in consideration the magnitude of the flows transferred from the Orinoco River (more than 320 m<fnref n="3" />·s⁻<fnref n="1" />), and the distance between the mouths of the Orinoco and Amazon rivers (about 1500 km).

<footnote n="1">¹ Géosciences Environnement Toulouse, Université de Toulouse, CNES, CNRS, IRD, UPS, 31400 Toulouse, France; alain.laraque@ird.fr</footnote>

<footnote n="2">² Instituto de Mecánica de Fluidos, Universidad Central de Venezuela—UCV, Caracas 1050, Venezuela; lopezjoseluis7@gmail.com</footnote>

<footnote n="3">³ Departamento Manejo de Bosques y Medio Ambiente, Facultad de Ciencias Forestales, Universidad de Concepción—UdeC, Concepción 407374, Chile</footnote>

<footnote n="4">⁴ Centro de Mecánica de Fluidos y Aplicaciones—CEMFA, Universidad Simón Bolívar—USB, Edificio Fluidos y Operaciones Unitarias, Caracas 89000, Venezuela; pgeorg@cantv.net</footnote>

<footnote>* Correspondence: syepez@udec.cl</footnote>

<footnote>†</footnote>

<footnote>This paper was written in tribute to our colleague and friend Paul Georgescu, who passed away shortly before this publication. He supported the development of this work and followed it step by step. As first author, I must thank Paul for inviting me to accompany him in this fantastic adventure in the Casiquiare, carried out in September 2000, in one of the least known and unexplored regions of the planet. Rest in peace, after an exciting existence as an explorer and adventurous scientist, navigating thousands of kilometers of waterways throughout South America.</footnote>
<figure><img src="image_203.png" /><figcaption>**Figure 1.** (A) Location map of the Casiquiare natural channel, connecting the Orinoco and Amazon basins. (B) Close view of the fluvial system showing main tributaries of the Casiquiare Channel. (C) Mean hydrographs of the Casiquiare channel (in Tamatama and Solano) and the Orinoco River in Tamatama, using discharge data from the period 1970–1986.</figcaption></figure>

The Casiquiare Channel is a very isolated watercourse (accessible only by river or air), located in an environment virtually devoid of riparian population but subject to some degree of insecurity due to illegal mining activities and guerrilla and paramilitary incursions. For these reasons hydrological measurements have been scarce, the latest being in September 2000 [1] and in December 2001 [2]. Figure 1 presents a close view of the fluvial system. The Upper Orinoco bifurcates downstream at the Tamatama village and runs for about 356 km before meeting Guainia River, which from this point on becomes the Negro River. The main tributaries of the Casiquiare River are all concentrated along its left bank (Pamoni, Pasiba, Siapa, and Pasimoni rivers) (Figure 1B). Figure 1C presents the mean hydrographs of Casiquiare in Tamatama and Solano, as well as the Orinoco in Tamatama, covering the same period of common discharge data between 1970 and 1986 (from The Global Runoff Data Centre—GRDC website [3]).

This paper aims to present: (a) a review and synthesis of the hydrological and sedimentological knowledge of this unique river; and (b) the first and unpublished hydro-sedimentary balance of the Casiquiare fluvial system at the bifurcation and at the mouth, based on the measurements made by an expedition that took place in September 2000 with the participation of Laraque and Georgescu, two of the authors of this paper [1]. The paper also presents a brief history of the discovery of the Casiquiare Channel and examines various theories concerning its origin. The problem of tracing the boundaries of the Casiquiare basin is also addressed.

## 2. Discovery of the Casiquiare Channel

Spanish conquistadors heard about the existence of the Casiquiare Channel from native people during the seventeenth century. The first account of this hydrologic connection is generally attributed to Acuña in the year 1641 [4]. Sanson Fer, cartographer of the King of Spain, presented a communication between the two rivers on their maps in 1707. Some documents in 1726 refer to trips of Portuguese traders on this channel [5]. Historical sources are also found in Humboldt's narrative [6]. The Jesuit priest Manuel Román, superior of the Spanish missions, registered the existence of a connection between the Orinoco and the Amazon rivers in 1744. The first scientific description of the Casiquiare Channel is attributed to La Condamine [7], but firmly denied at the Academy of Sciences in Paris by Philippe Buarche, the royal geographer, who described Casiquiare as a geographical monstrosity. The waterway was not officially referred to by the name of the Casiquiare Channel until 1772 in the so-called "Plan Geográfico de Santa Fe de Bogota, Nuevo Reino de Granada" signed by the cartographer Joseph Aparicio Moreta. However, this is not totally accepted by the scientific community
until 1800, when Alexander von Humboldt and Aimé Bonpland landed in Venezuela. They navigated the Casiquiare Channel on 10 May 1800, and closed the controversy over the existence (or not) of a communication between the Amazon and the Orinoco [6]. From this moment, the existence of the Casiquiare Channel has been definitively accepted by the modern science community. Figure 2 shows a map presented by Humboldt indicating the course of the river channel between the Orinoco and Negro rivers [8]. Most recently in 1943, during World War II, the U.S. Army Corps of Engineers carried out the first hydrologic study for the navigability in this channel in order to develop a secure route (away from the German submarines navigating along the South American coast off the mouth of the Amazon) for transportation of latex between Brazil, Colombia, and Venezuela, to be used for war purposes [9].

<figure><img src="image_204.png" /><figcaption>**Figure 2.** Map presented by Humboldt in 1812 showing the course of the Casiquiare Channel between the bifurcation of the Orinoco and the confluence with the Negro River. Used by permission of Société de Géographie, from Georgescu et al. [10].</figcaption></figure>

**3. General Description**

The description and geographical location of the "hydrological singularity" of the Casiquiare Channel is mentioned in several articles on the hydrology of the Orinoco and Amazon basins [4,11,12]. However, scientific knowledge of the Casiquiare Channel is generally limited in time [13–15], subjective, incomplete, approximate, and often erroneous because it results largely from summary observations, without the support of physical measurements. No hydro-sedimentary balance of a complete hydrological cycle has yet been carried out.

Figures 3 and 4 show the morphological differences between the Orinoco bifurcation which coincides with the beginning of the Casiquiare Channel and the confluence of the latter with the Guainia River. This bifurcation begins abruptly and perpendicularly to the drainage axis of the Orinoco River (Figure 1B) just downstream of the locality of Tamatama. The Casiquiare Channel turns southwest to meet the Guainia River from the northwest at the end of its stream to form the Negro
River, just upstream of San Carlos locality. Further downstream, the Negro River meets Rio Solimões to form the Amazon River near the town of Manaus in Brazil. The coordinates of the bifurcation at its origin, in the Upper Orinoco in Venezuela are N. 03° 08′ 25.26″ and W. 65° 52′ 48.66″, and the coordinates of its mouth, also in Venezuela are N. 02° 00′ 15.18″ and W. 67° 06′ 52.44″.

<figure><img src="image_205.png" /><figcaption>**Figure 3.** Satellite views of the Casiquiare Channel: (A) at the bifurcation, and (B) at the confluence, showing location of transects T1 to T6 (images from Google Earth).</figcaption></figure>

<figure><img src="image_206.png" /><figcaption>**Figure 4.** Aerial photos of the bifurcation (A) and confluence (B) taken from a helicopter by A. Laraque on September 10, 2000.</figcaption></figure>

The Casiquiare Channel, bordered by gallery forests, is 356.4 km long and its width varies from 46 to 610 m. Minimum water depths are about 0.30 m upstream and 1.2 m downstream. Between its two extremities, the elevation difference is about 21 m, with an average slope of 6 cm·km⁻¹ [9]. But this hydraulic gradient can vary from 7.82 to 8.55 cm·km⁻¹ between low and high water. The surrounding plain has a slope of 10.3 cm·km⁻¹.

On the Casiquiare left bank, the contribution of the Pamoni, Pasiba, Siapa, and Pasimoni rivers come from the Sierras Curupira, Tapirapeco, Imeri, and Neblina located in southern Venezuela, with peak elevations between 2000 and 3000 m.a.s.l., the highest of which reaches 3014 m (Pico da Neblina in Brazil). The land drained by the left bank of the Casiquiare and by the Upper Orinoco is composed of undifferentiated Proterozoic rocks [16]. This mountain range with high rainfall rates, about 4000 mm·yr⁻¹, constitutes a real water reservoir that feeds the left bank of the Casiquiare Channel. This northwest-facing runoff reflects the slight slope in the same direction, which drains the plains surrounding the Guyanese shield between 150 and 200 m.a.s.l., where some steep residual landforms are observed at about 500 m.a.s.l. [14].

**4. Origin and Evolution of the Casiquiare Channel**

Geologists agree that the capture of the Orinoco by the Casiquiare River is more recent than the formation of the Andean range which has profoundly modified the direction of circulation of the Amazon and Orinoco rivers. According to ichthyological observations, Goldstein, [17] attributes the
birth of the Casiquiare Channel to the capture of a secondary channel of the Orinoco River on the Pleistocene-Holocene limit, about 10,000 years ago.

Stern [18] was the first one to describe the genesis of the Casiquiare. Vareschi [19] mentioned an incomplete capture by overflow and regressive erosion produced by a northern affluent of the Negro River. This is one of the largest examples of natural hydrological capture in the world. The mechanism of capture, the current situation, and future trends are illustrated in Figure 5 [18,19]. At remote times the Casiquiare did not exist. The Orinoco River passed in two channels around a fluvial island denoted by I (Figure 5-1). The Upper Casiquiare used to be a branch of the Orinoco River. The confluence of the Cunucunuma River (CU) with the Orinoco (O) was located at the north branch (S) of the island. The lower reach of the south branch was Caño Seco (CS), which used to flow around Quiritari island (Q) before entering the main channel (O) of the Orinoco River. At some distance to the south, the Pamoni River, a perennial river, was the headwaters of the river today called Casiquiare, which flowed into the Negro River and the Amazon basin. The approximate drainage division between the Orinoco and Amazon basins used to be the dashed line denoted by D (Figure 5-1).

<figure><img src="image_207.png" /><figcaption>**Figure 5.** Sketch-maps of the Casiquiare fluvial system showing past and future trends in the evolution of the channel, according to Stern [18] as illustrated by Vareschi [19]. O = Orinoco; C = Casiquiare; CU = Cunucunuma River; B = bifurcation; I = fluvial island; CS = Caño Seco; BA = Buenos Aires sector; V = Venados (abandoned town); P = Pamoni River; S = north channel of the island; Q = Quiritari Island (in orange); D = water divide; a = small channel; b = small channel.</figcaption></figure>

Probably, during high flows, the south branch of the Orinoco River began flowing to the Pamoni River through channel (a) and channel (b). The alluvial material composing the drainage divide (D) was eroded and a new channel was formed, which was the genesis of the Casiquiare River by avulsion processes. The capture of the Orinoco waters was helped by the existence of a terrain slope larger toward the south than toward the west. Caño Seco was abandoned and became a dry channel, which flows only during the high flow season. Near an abandoned town, called Venados site (V), the Casiquiare is almost dry during the low season, making navigation difficult (Figure 5-3). This observation was made during the Oriampla Expedition to the Orinoco in January 1980 [10]. At present time, during a combination of high flows in the Cunucunuma River and low flows in the Orinoco, the flow in Caño Seco (CS) could revert as it is shown in Figure 5-5.
Stern [18] reported that the flow discharge, flow depth and width in the upstream reach of the Casiquiare Channel had increased in the last 50 years, according to stories from old pilots, navigators, and local inhabitants. This is reinforced by the accumulation of sediment and point bars in the Orinoco River downstream from the point of bifurcation. Stern suggested that in a future time the Casiquiare will totally capture the Orinoco flows and that the headwaters of the Orinoco would be the headwaters of the Cunucunuma River (Figure 1B). Although a few authors like [20] questioned the Stern theory of the Casiquiare formation, Vareschi [19] supports the opinion of Stern based on field observation of two abandoned channels connecting the Orinoco with the Casiquiare, noting that their channel widths were significantly smaller than the Casiquiare width at the present time.

Using a geomorphometric analysis of the region with Digital Elevation Models, Hypsometry, Isobases and Swath Profiles, Grohmann et al. [21] explain the capture of the Orinoco River and the formation of Casiquiare canal by tectonic controls.

A recent study by Stokes at al. [22] used measurements from the 1943 U.S. Army Corps of Engineers and hydraulic calculations to show that the Rio Casiquiare is actively capturing the Upper Orinoco River, driven by a combination of in-channel sedimentation in the Orinoco and seasonal inundation of the drainage divide that favors the steeper Casiquiare River. They consider that the ongoing reorganization of the river system, is under tectonic controls and also conclude that the Casiquiare River course will eventually become the dominant channel, which confirms Stern's pioneering studies [18,23].

To conclude this paragraph, it seems necessary to clarify that the location of the initial capture of the Orinoco is located at the level of the south branch and downstream of the river island "I" in Figure 5-1-5, and not at the bifurcation of the Casiquiare, which is located upstream of the same river island and downstream of Tamatama, as is too often mentioned in the literature. Indeed, at this point in past time, it was a simple division of the Orinoco River into two branches that bypassed the river island "I". The critical area of the initial avulsion that diverted the south branch of the Orinoco is located at Caño Seco (Figure 5-1-5). Subsequently, in the event of a continuation of this river capture, the northern branch of the Orinoco, which bypasses the river island, will dry up and so will the Caño Seco. At that time the capture will be total and the upper Orinoco River will be completely diverted towards the Amazon basin (Figure 5-6).

# 5. Data and Methodology

## 5.1. *Sampling and Measurements*

During the September 2000 expedition, bathymetric, discharge and physico-chemical profile measurements were made along six transects denoted by T1 to T6 (Figures 3 and 6), using a small boat provided with an echo-probe (Eagle Strata 128) coupled with a GPS (GARMIN, Etrex model; Ap = +/- 1m), a turbidimeter (AQUALITYC; Ap = +/- 0.01 NTU), a thermometer, a portable pH meter (WTW pH 320; +/- 0.01), and a conductivity meter (WTW LF 318; Ap = +/- 0.1 µS·cm⁻¹). The physico-chemical measurements were made at three points on the vertical (surface, middle, and bottom) at the center of the river. Total suspended sediment (TSS) was also collected with a 500 mL sampler. For discharge measurement, first, a bathymetric survey is carried out using the echo sounder and GPS to obtain the flow area at each respective section of the river. The flow velocity is determined using the boat (canoe) as a drifting float on three longitudinal lines distributed at 25%, 50%, and 75% of the bank distance of the section. GPS provides us the drift speed of the canoe, and therefore the surface velocity. Thus, the average velocity of the flow section is calculated by applying a correction coefficient between 0.8 and 0.95 [24], generally accepted for this type of flat watercourse. Water discharge was determined as the product of the velocity and the area, and material flows (in kg·s⁻¹) were obtained by multiplying the average values of the measured TSS concentrations (in mg·L⁻¹) with the discharge (in m³·s⁻¹).
<figure><img src="image_208.png" /><figcaption>**Figure 6.** Bathymetric profiles obtained at 6 different sections in the bifurcation and in the confluence of the Casiquiare Channel. T1: Orinoco upstream of bifurcation; T2: Orinoco downstream of bifurcation; T3: Casiquiare at inlet; T4: Guainia River before confluence; T5: Casiquiare at outlet; T6: Negro River at San Carlos. Depths and widths are given in meters. See Figure 3 for location of cross sections collected in the 2000 expedition.</figcaption></figure>

## 5.2. Analytical Techniques

Water samples were filtered with filters of acetate (cellulose) with a porosity of 0.45 µm for the total suspended sediment (TSS). After being dried, these fractions were weighed to a precision of 0.1 mg (Sartorius scale). In the laboratory, 100 mL of filtrates were heated in a steamer at 105 °C to obtain the dry residue, which corresponds to the sum of total dissolved solids (TDS) and the total dissolved organic matter (DOM).

## 5.3. Morphometric Analysis

A morphometric analysis was carried out to accurately check the contributing area of each basin, including a description of the elevation and slope changes both of the Upper Orinoco basin and the Casiquiare Canal basin. For this, we used the SRTM (Shuttle Radar Topography Mission)—Digital Elevation Model (DEM) of the Nasa at 1 arc of a second, with approximately 30 m [25] in combination with the Fluvial Corridor Toolbox software [26]. Thus, a synoptic diagram was created for each basin. From this set of digital cartographic data in three coordinates (XYZ) or DEM, “elevation and slope tool” (in Fluvial Corridor Toolbox) enables the extraction of topographical data such as the elevation and the slope of a set of polylines (e.g., a hydrographic network). It is based on the add surface information (3D Analyst Tools) and can be applied over a linear network. The “Z-mean” elevation was calculated over the entire segment. Finally, the “slope” field is the ratio between the upstream and downstream difference of elevation and the segment length. The drained area at the points is also important information for the dynamics of the river system. Therefore, the “watershed tool” was also used to determine the drainage area along a continuum. It is based on the extraction of data included in a flow accumulation raster, provided by the user and derived from a DEM [26].

# 6. Results

The hydrologic data (maximum depth, width of section, and surface stream velocity) and physical-chemical parameters (T°, electrical conductivity, pH, turbidity, and suspended sediment concentration) are presented in Table 1 for each section in consideration. Figure 6 presents the bathymetric profiles at the three cross sections of the Orinoco-Casiquiare's bifurcation and at the three cross sections of its confluence with the Guainia River to form the Negro River. Comparisons of these data and calculations of hydro-sedimentary budgets at the three measuring points of the bifurcation and confluence of the Casiquiare Channel provide a better understanding of its hydro-geodynamic functioning.
<table><caption>**Table 1.** Hydrologic and physico-chemical data obtained from measurements at the bifurcation and confluence of the Casiquiare Channel in September 2000. Q = flow discharge; V = surface flow velocity; W max = maximum channel width; H max = maximum depth; Temp. = temperature; EC = electrical conductivity; Turb = turbidity; TSS = total suspended sediment concentration.</caption><thead><tr><th>Transect Number</th><th>Date</th><th>Station</th><th>River</th><th>Q m<sup>3</sup> s<sup>-1</sup></th><th>V m s<sup>-1</sup></th><th>W Max m</th><th>Depth of Sampling m</th><th>H Max m</th><th>Temp °C</th><th>EC µS-cm<sup>-1</sup></th><th>Turb NTU</th><th>pH</th><th>TSS mg L<sup>-1</sup></th><th>TSS Fluxes kg s<sup>-1</sup></th></tr></thead><tbody><tr><td rowspan="2">T1</td><td>9-09-00</td><td>Puerto Ayacucho</td><td>Orinoco</td><td rowspan="2">2323</td><td>1.94</td><td rowspan="2">380</td><td>0</td><td>0</td><td>27.3</td><td>7.0</td><td>26.0</td><td>5.00</td><td>49.20</td><td rowspan="2">79.69</td></tr><tr><td>10-09-00</td><td>Tamatana</td><td>Orinoco</td><td>-2</td><td>5.20</td><td>26.0</td><td>14.0</td><td>22.0</td><td>5.74</td><td>34.40</td></tr><tr><td>T2</td><td>10-09-00</td><td>Downstream</td><td>Orinoco</td><td>1343</td><td>1.11</td><td>270</td><td>0</td><td>5.80</td><td>26.5</td><td>15.0</td><td>24.0</td><td>5.70</td><td>32.00</td><td>42.99</td></tr><tr><td rowspan="2">T3</td><td>10-09-00</td><td>Bifurcation</td><td>Casiquiare</td><td>720</td><td>1.39</td><td>88</td><td>0</td><td>7.30</td><td>26.0</td><td>14.0</td><td>18.0</td><td>5.87</td><td>43.60</td><td>31.66</td></tr><tr><td>"</td><td>Bifurcation</td><td>"</td><td>-3</td><td>-6</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>30.0</td><td>34.0</td><td>44.40</td><td></td></tr><tr><td rowspan="2">T4</td><td>11-09-00</td><td>Mouth</td><td>Casiquiare</td><td>5438</td><td>1.39</td><td>320</td><td>0</td><td>14.10</td><td>26.7</td><td>7.0</td><td>7.9</td><td>4.20</td><td>8.00</td><td>48.94</td></tr><tr><td>"</td><td>Confluence</td><td>"</td><td>-12</td><td>-6</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>7.0</td><td>7.9</td><td>10.00</td><td></td></tr><tr><td>T5</td><td>11-09-00</td><td>Confluence</td><td>Guainia</td><td>2573</td><td>0.83</td><td>380</td><td>0</td><td>11.30</td><td>26.8</td><td>14.0</td><td>0.4</td><td>3.73</td><td>9.00</td><td>0.51</td></tr><tr><td>T6</td><td>11-09-00</td><td>San Carlos</td><td>Negro</td><td>8034</td><td>1.19</td><td>600</td><td>0</td><td>14.30</td><td>26.9</td><td>9.0</td><td>1.5</td><td>3.20</td><td>0.20</td><td>5.60</td></tr><tr><td>T7</td><td>12-09-00</td><td>San Gabriel</td><td>Negro</td><td>6878</td><td></td><td></td><td>0</td><td></td><td>28.0</td><td>10.0</td><td>0.9</td><td>3.48</td><td>3.40</td><td>44.99</td></tr></tbody></table>
## 6.1. Morphology Features and Hydro-Sedimentary Budget at the Bifurcation

While the Orinoco River has a width of 380 m and 270 m, respectively, at T1 and T2 (see Figure 6), before and after the bifurcation, the width of the Casiquiare Channel is smaller (88 m) and has a greater depth (7.30 m) than the Orinoco River (5.20 m). Flow velocity increases slightly from $1.28 \text{ m} \cdot \text{s}^{-1}$ in the Orinoco River (upstream) to $1.39 \text{ m} \cdot \text{s}^{-1}$ at the entrance of the Casiquiare (Table 1). The section at the entrance of the Casiquiare corresponds to about 28.5% of the section of the Orinoco before the bifurcation (Figure 6). The water balance is nearly 90% complete. Indeed, the sum of the flows at the Orinoco downstream of the bifurcation (1343 $\text{m}^3 \cdot \text{s}^{-1}$) and the one at the entrance to the Casiquiare (720 $\text{m}^3 \cdot \text{s}^{-1}$) is 2063 $\text{m}^3 \cdot \text{s}^{-1}$ instead of 2324 $\text{m}^3 \cdot \text{s}^{-1}$ for the Orinoco upstream of the bifurcation. It is possible that this inaccuracy is due to the flow measurement protocol. At the time of the study, the Casiquiare diverts about 30% of the flow of the Upper Orinoco River.

According to the annual averages calculated over the period 1970–1975, the bifurcation with a discharge of $320 \text{ m}^3 \cdot \text{s}^{-1}$ diverts 25% of the flow from the Upper Orinoco [11]. Ron et al. [27] report that the minimum and maximum flows at the entrance to the Casiquiare, 127 and 680 $\text{m}^3 \cdot \text{s}^{-1}$ respectively, maintain the same proportion between 22% and 25% of the flows in the Upper Orinoco River. These two rivers obviously have the same hydrological behavior with flow differences by a factor of 5 between rising and falling stages.

With regard to the physico-chemical data, we obtained pH, electrical conductivity, and turbidity values of the Orinoco water, respectively in the order of 5.80, $14 \text{ µS} \cdot \text{cm}^{-1}$ at 25 °C and 25 NTU for TSS concentrations of $35 \text{ mg} \cdot \text{L}^{-1}$ (Table 1). Note a higher value in TSS at the entrance to the Casiquiare (44 $\text{mg} \cdot \text{L}^{-1}$) probably due to the increase in velocities, which results in a resuspension of particulate matter. In terms of sediment fluxes, the bifurcation balance closed to nearly 94%. Indeed, the sum of the sediment flows of the Orinoco downstream of bifurcation and the one at the entrance of Casiquiare is $75 \text{ kg} \cdot \text{s}^{-1}$, very close to $80 \text{ kg} \cdot \text{s}^{-1}$ for the Orinoco upstream of the bifurcation. A report of the field work made in December 2001 [2] mentioned TSS concentrations around $20–25 \text{ mg} \cdot \text{L}^{-1}$, composed of 20% or $5 \text{ mg} \cdot \text{L}^{-1}$ of sand ($> 62 \text{ µm}$) and 80% or $15–20 \text{ mg} \cdot \text{L}^{-1}$ of fine TSS (between 62 and 0.45 µm).

## 6.2. Morphology Features and Hydro-Sedimentary Budget at the Confluence

At the confluence with the Guainia, the Casiquiare Channel presents widths, maximum depths, flow velocities, and discharges of 320 m, 14.10 m, 1.39 $\text{m} \cdot \text{s}^{-1}$, and 5439 $\text{m}^3 \cdot \text{s}^{-1}$, respectively, compared to 380 m, 11.3 m, 0.83 $\text{m} \cdot \text{s}^{-1}$, and 2573 $\text{m}^3 \cdot \text{s}^{-1}$ for the Guainia River, and 600 m, 14.30 m, 1.19 $\text{m} \cdot \text{s}^{-1}$, and 8034 $\text{m}^3 \cdot \text{s}^{-1}$ for the Negro River just after the confluence (Table 1).

It should be noted that the measurements carried out in September 1970 yielded similar results [28]. At the bifurcation they reported $2430 \text{ m}^3 \cdot \text{s}^{-1}$ for the Orinoco and $646 \text{ m}^3 \cdot \text{s}^{-1}$ at the entrance of the Casiquiare. At the outlet, they measured $5440 \text{ m}^3 \cdot \text{s}^{-1}$ at the Casiquiare and $3336 \text{ m}^3 \cdot \text{s}^{-1}$ at Guainia. The mean flow discharge is about $2800 \text{ m}^3 \cdot \text{s}^{-1}$ according to water levels recorded during the period 1970–1975 [2]. Using sixteen years of available discharge data during the period 1970-1986 (from the GRDC website), the hydrographs in Figure 1C were constructed, which illustrates comparatively the monthly variations of: (i) the Orinoco River discharge in Tamatama, (ii) the Casiquiare in its mouth downstream of Tamatama and (iii) in its mouth at the Solano hydrometric station. These last two hydrographs provide information on the extent of the contributions of the tributaries of the left bank of the Casiquiare.

At their confluence, Casiquiare and Guainia have morphologically similar sections (Figure 6). The flow of Casiquiare is two times higher than the one of the Guainia, mainly due to its 1.67 times higher flow velocity compared to that of the Guainia. The water balance is nearly 100% complete. Indeed, the sum of the flows measured at the outlet of the Casiquiare and at Guainia is $8011 \text{ m}^3 \cdot \text{s}^{-1}$, compared to $8034 \text{ m}^3 \cdot \text{s}^{-1}$ for the newly formed Negro River (see T6 in Figure 3B).

With regard to physical-chemical data, the pH, electrical conductivity, turbidity, and TSS concentrations of the three confluence points are respectively: 3.73; 14 µS·cm⁻¹; 0.4 NTU; 0.2 mg·L⁻¹
for Guainia, 4.2; 7 µS·cm⁻¹; 7.6 NTU; 9 mg·L⁻¹ for Casiquiare and 3.9; 9 µS·cm⁻¹; 1.5 NTU; 5.6 mg·L⁻¹ for Negro, values similar to those found by Vegas et al. [29].

The electrical conductivity of Guainia River is twice as high as Casiquiare, probably due to a high presence of organic anions from humic and fulvic forest acids, which make them more acidic. Indeed, the black waters of the Guainia River, typical of "black water rivers", have just crossed a huge forest plain and are therefore much depleted of TSS compared to those of the Casiquiare, part of which comes from the loaded TSS waters of the Upper Orinoco.

The balance of sedimentary flows at the confluence is nearly 91% complete. Indeed, the sum of the flows from the Casiquiare and Guainia is 49.5 kg·s⁻¹ instead of 45 kg·s⁻¹, for Negro River in San Carlos station, just downstream of the confluence (see T6 in Figure 3B). Given the approximations on the flow calculations obtained from the adopted summary and simplified protocol, both from the point of view of flow measurement and from that of the calculations of the average concentrations of TSS, it is reasonable to assume that the law of conservation of mass is respected, both at the bifurcation and at the confluence.

### 6.3. Global Budget and Functioning of the Casiquiare Hydraulic Complex

The flow area of the channel at the outlet is 7.5 times larger than the one at the inlet, due to maximum depths and widths which are, respectively, 2 and 3.6 times greater. However, the flow velocity remains constant along the channel (1.39 m·s⁻¹). These flow velocities are higher than those of the Orinoco upstream and downstream of the bifurcation (1.28 and 1.11 m·s⁻¹), and that those of the Guainia (0.83 m·s⁻¹) and of the Negro River (1.19 m·s⁻¹).

These higher Casiquiare flow velocities would come from both its elevation difference and the hydrological thrust from about 100 tributaries, mainly on the left bank, which input a total flow of 4720 m³·s⁻¹, i.e., 6.6 times higher than those at the entrance to the channel.

The flow discharge at the outlet is 7.5 larger than at the entrance, but its sedimentary fluxes are only 1.5 times higher, due to TSS concentrations 4.9 times lower at the end of the channel. The travel time of the waters of the Casiquiare, from its entry to its exit, is about 3 days during the rising stage of the hydrograph. Eight thresholds define its course and make it difficult to navigate in the falling stage, when many rocks are exposed at the surface of the water.

The values of turbidity, electrical conductivity, and pH decreased from 24 to 7.6 NTU; 14 to 7 µS·cm⁻¹ and 5.87 to 4.2, respectively, from the inlet to the outlet. This longitudinal variability on pH and EC had already been highlighted by Edwards and Thornes [14]. Thus, the white waters at the entrance of the Casiquiare, which correspond to those of the Upper Orinoco, loaded with TSS, acquire, during their travel, more acidic pHs, turbidity, and lower conductivity, due to lateral inputs that have drained the forest cover and swampy plains on its left bank. In these ecosystems, there is little erosion due to lack of relief and slow runoff allows water to soak up dissolved organic matter. A progressive dilution of the waters of the Casiquiare then gives it a facies more and more similar to that of the "black rivers".

Figure 7 presents a schematic summary of the main parameters measured in the fluvial system. It represents the first model of the hydro-sedimentary behavior of the Casiquiare Channel with a distribution of morpho-hydrodynamic parameters in the three sections of its bifurcation and the three sections of its confluence.

Lateral sediment fluxes have been obtained from mass conservation principles. The TSS flux is equal to 17 kg·s⁻¹ and the corresponding mean concentration is 3.6 mg·L⁻¹ (Figure 7C). Similarly, the electrical conductivity, pH, and turbidity of lateral inputs are equal to 7.15 µS·cm⁻¹, 3.42, and 4.4 NTU (Figure 7B). These values are typical of the "black water" rivers in the Amazon basin.

A relationship between the TSS (mg·L⁻¹) and the turbidity (NTU) was obtained from the measurements in the Casiquiare Channel hydrological complex (Figure 8):

$$TSS = 1.4975 \times \text{Turb} + 0.6206 \quad (1)$$
<figure><img src="image_209.png" /></figure>

**Figure 7.** Schematic diagrams of the main morpho-dynamic and physico-chemical parameters measured in the Casiquiare hydrological complex from 9 to 12 September 2000. Numbers inside rectangles indicate flow discharges (Chart A), electrical conductivity (Chart B), suspended sediment concentrations (Chart C) and sediment fluxes (Chart D).

<figure><img src="image_210.png" /></figure>

**Figure 8.** Relation between TSS (mg·L⁻¹) and turbidity (NTU) for the Casiquiare Channel.

It is worth noting that the interannual discharge of the Casiquiare bifurcation (320 m³·s⁻¹) corresponds to only 0.85% of the interannual discharge of the Orinoco at its delta, estimated at 37,600 m³·s⁻¹ [30–32]. In its mouth, the Amazon River presents an interannual discharge of 209,000 m³·s⁻¹ [33],
of which 0.16% comes from the Upper basin of the Orinoco, and not 1.50% as reported by some authors such as Palacios [34], who takes into account the Casiquiare outlet discharge, instead of the inlet discharge at its bifurcation.

## 7. The Problem of Tracing the Boundaries of the Watershed

Tracing the boundaries between the watersheds of the Casiquiare and Upper Orinoco is an insoluble problem with the concepts of classical hydrology, since depending on the season, 20% to 30% of the waters of the Orinoco above the bifurcation flows into the Casiquiare. Thus, the Upper part of the Orinoco watershed feeds two basins and it is a kind of diffuse area which is not well defined.

At its bifurcation and confluence with the Negro River, the problem arises because fuzzy boundaries cross the marshy plains characterized by shallow lagoons and flooded forests under which superficial two-way laminar-flow runs according to the seasons and to the water levels of the rivers they connect. In addition to this, there are small secondary canals connecting the Orinoco to the Casiquiare and the latter with the Guainia River, mentioned by Georgescu [8], Carreteiro [35], and CAF [36]. These features are found in the depressions and vast plains of major river basins, such as the Congo [37]. Another peculiarity of the Casiquiare watershed is that it is limited almost exclusively to the north-west by its own river course. Indeed, the tributaries of its right bank are small streams, very few in number, whose drained areas are negligible compared to that of the whole basin.

Lewis and Weibezahn [38], Lewis and Saunders [39], Paolini [40], and Brown et al. [41] present acceptable boundaries between the two largest basins on the continent, although the ambiguity of the surface area of the Upper Orinoco River that feeds both this river and the Casiquiare is not mentioned.

According to Ron et al. [27], the area sensu stricto (s.s.) of the Casiquiare watershed, i.e., the area whose rainwater flows directly and solely towards the Casiquiare Channel, is about 33,000 km², with an average flow rate at its outlet of 2790 m³·s⁻¹ [36] or 2100 m³·s⁻¹ [42]. According to the same author, the area of the Upper Orinoco River above the bifurcation is 37,870 km². However, in this study a morphometric analysis using the Fluvial Corridor Toolbox and the DEM SRTM at 30 m showed that the area of the Upper Orinoco River Basin presents a contributing area above the bifurcation of 40,110 km², coinciding quite well with the 40,000 km² estimate made by Stokes et al. [22]. Likewise, for the Casiquiare Channel basin a contributing area of 42,810 km² was estimated (Figure 9). Since it supplies both the Orinoco downstream of the bifurcation and the Casiquiare (in the order of 25% of its drainage water), it can be deduced that a total area of 82,920 km² contributes to the Casiquiare water supply.

For this reason, the specific flow rate of the Casiquiare basin cannot be calculated, because its catchment is composed of two parts. The first one (42,810 km²) flows directly to the Casiquiare Channel (it is the sensu stricto (s.s.) watershed of the Casiquiare Channel) and the second one (40,110 km²) flows at the same time to the Orinoco and Casiquiare. That means that the Casiquiare basin is delimited by both a typical watershed line (surrounding an area of 42,810 km²) and by an unusual watershed area of 40,110 km² (which corresponds to the Upper Orinoco catchment) shared by the Orinoco and Amazon basins (Figure 9). The addition of these two contours delimits the surface area of the basin sensu lato (s.l.) of the Casiquiare, including the famous shared surface of the Upper Orinoco which does not only flow to the Casiquiare.

Figure 9 also compares the calculated slopes with the DEM SRTM (pixel ~30m) for the Casiquiare Channel with respect to the slope calculated locally by USACE [9], small differences can be observed (8.5 versus 6 cm·km⁻¹). These small differences can be explained as a result of uncertainties in local sampling (elevations using barometric measurements surveyed by the USACE), as well as by the low spatial resolution of the DEM used (in the current study). While terrain elevation models are very helpful for direct quantification of slope in channels, the accuracy of these analyses is a function of the spatial resolution of the DEM, the data acquisition system, the smoothing algorithms, as well as the methodology used for channel profile extraction. Currently, with the use of DEMs derived from Light Detection and Ranging (LIDAR) data, it is possible to obtain accurate slope estimates over relatively short distances. However, considerable shortening of channels and excessively steep
slope estimates have also been documented when using a coarse data system such as ASTER GDEM 30 m or similar [43]. When evaluating the profiles of both channels a very subtle slope break can be observed that effectively shows the occurrence of the bifurcation in the main channel from the Upper Orinoco to the Casiquiare Channel. However, in order to better understand this hydrological phenomenon in the future it will be crucial to acquire and analyze new field measurements with the most recent technologies.

<figure><img src="image_211.png" /><figcaption>**Figure 9.** Boundaries of the Casiquiare s.s. catchment and Upper Orinoco catchment (green lines). The blue line delineates the Upper Orinoco River and the red line the Casiquiare Channel with a total length of ~350 km. A synoptic diagram highlights the contributing area in each basin, as well as the elevation and slope of each of the two rivers.</figcaption></figure>

Depending on the season, 20 to 30% of the water from the Upper Orinoco upstream of Tamatama flows into the Casiquiare, which means that the same percentage of rainfall on the part of the catchment area upstream of the bifurcation is discharged through this channel, while the rest (70–80%) flows into the Orinoco River. Thus, it can be assumed that the same portion of a watershed feeds two rivers unevenly in the proportions previously stated.

This natural situation is unique in the world in terms of: (i) its location at the headwaters of a watershed, (ii) the areas of the sharing basin, (iii) discharge involved, (iv) for flows in only one direction, (v) the length of the channel, and (vi) the distance between the mouth of the Amazon and Orinoco rivers that are fed by the Upper Orinoco River.
## 8. An Ecological Connectivity Between the Orinoco and Amazon Basins

The Casiquiare links two basins characterized by different geomorphologic and geochemical conditions. The upper basin of the Orinoco River is a typical mountain relief of the tropics, covered with dense vegetation, containing mostly clear-water streams with slightly acid pH and moderate concentrations of dissolved organic and inorganic substances [44]. On the other hand, the upper basin of the Negro River, at lower elevations and covered in tropical forests, with areas of savanna occurring in upland regions, is dominated by 'blackwater streams' with high transparency but strong staining by tannins and other organic compounds leached from vegetation, extremely low pH, negligible amounts of solutes, and substrates of fine quartz sand [42].

Above the Casiquiare bifurcation, the Orinoco is a white-water stream with a common light brown color, so the Casiquiare begins its course with the same water conditions. However, during its ~350 km long-course, along its left bank, the Casiquiare receives the flows of many predominant black-water streams, like the Pamoni, Pasiba, Siapa, and Pasimoni rivers, carrying clear-acid-swamp water due to the high content of dissolved-organic humic acids. Thus, the Casiquiare suffers a gradual transition in the color of the river until its discharge into the Negro River, representing a hydrochemical gradient between white waters at its origin and black waters at its mouth.

The Casiquiare influences the migration of aquatic organisms between the Orinoco and Amazon basins and seems to function as an interbasin dispersal corridor for fish, but the effectiveness of this connection is mitigated by the strong physical-chemical and ecological gradient that spans its length. Winemiller and Willis [45] concluded that the degree to which the river serves as a dispersal corridor or barrier is variable and depends on the physiological and ecological tolerances of individual species.

## 9. Other River Captures

Very few natural hydrological captures exist worldwide that present some similarities with the Casiquiare. Without being exhaustive, it can be mentioned the Echimamish River in Canada, which connects the Hayes and Nelson Rivers flowing into Hudson Bay [46]. In this small connection that crosses a marshy region, the weak currents in the Echimamish River are reversed according to the water levels of the two connected rivers. This is the case for most of the world's bifurcations, many of which are intermittent and others are inside the same river basin. Another analogue ongoing river capture is the Selinda spillway of Southern Africa, which diverts water from the Okavango towards Kwando and Zambezi rivers, probably by overflow during the rising stage of Okavango River [47].

It seems that the only other current bifurcation similar to the Casiquiare is the Tarendö-elf canal in Sweden, which diverts 80 m³.s⁻¹ of the flows of the Torne River towards the Kalix River, along a winding path of 52 km in a marshy terrain. Its origin would also come from an overflow of the Torne River, which would be the cause of its partial capture. Two hundred km downstream of this junction, the Torne and Kalix rivers flow into the Bay of Bothnia in the Baltic sea, with only 20 km distance between its rivers mouths [48]. However, the hydrological singularity of the Casiquiare is unique in its size. It is the only permanent and one-way bifurcation that extends for more than 300 km long with flow discharges varying between several hundred and a few thousand m³.s⁻¹, it also connect two neighboring basins with very distant deltas separated about 1500 km apart.

## 10. Conclusions

The Casiquiare Channel is a bifurcation flowing all year in the same direction from the Orinoco basin to the Amazon basin. Throughout its course, this natural channel undergoes significant morphological, hydrological, and bio-geochemical variations, whose most visible witnesses are the increase in width (3 to 4 times), flow (7 to 9 times), and its change in water color (white to black), under the influence of tributaries coming from vast forest plains. For this reason, it can be called a chameleon channel.
Studies made by other researchers [2,9] were interested more in the hydrology and hydraulics of the system, than in the sediment balance. Water and sediment budget is important in the Casiquiare system because of future plans for establishing a South American river integration system through the Orinoco, Casiquiare, Negro, Amazon, Paraguay, Parana, and Río de la Plata Rivers [36]. In this paper, a hydro-sedimentary balance of the Casiquiare fluvial system has been presented based on field data collected in September 2000.

At the beginning of the falling stage of the hydrograph, the flow at the bifurcation inlet channel corresponds to about 13% of those of the outlet flows. The latter, in the order of 5500 m³·s⁻¹ or an annual average of 2800 m³·s⁻¹, are respectively 7 to 9 times higher than those at the inlet. The relatively high average slope (6 cm·km⁻¹) of the channel leads to relatively high velocities of up to 1.4 m·s⁻¹ during rising stage. The natural slope and these velocities produce a river channel without many meanders and its operating dynamics should be more one of erosion-transport rather than sedimentation.

The geochemical facies of the waters range from “white water” (pH = 5.87) loaded in suspended sediment in the order of 44 mg·L⁻¹, coming from the relief belt that delimits the Upper Orinoco basin with that of the Amazon, to a facies characteristic of the “black rivers”, more acidic (pH = 4.20) and less loaded by sediment (8 mg·L⁻¹) and dissolved mineral matter. This is due to significant dilution by numerous inputs from left bank tributaries crossing vast forest plains. Additionally, the electric conductivity changed from 14 to 7 µS·cm⁻¹. The longitudinal physical-chemical gradients clearly illustrate the evolution of this chameleon channel where the outlet waters are mixed by lateral inflows (tributaries), which are respectively 6 to 8 times higher than those at the inlet, depending on whether one is in rising stage or whether one considers the average module.

The surface of the Casiquiare catchment area sensu lato (s.l.) is about 82,920 km², and approximately half of it feeds two rivers at the same time, the Orinoco and Casiquiare. Thus it can be considered that only about 1/4 of the drainage area of the Upper Orinoco (40,110 km²) is flowing to the Casiquiare. It is an unusual watershed area shared between the Orinoco and Amazon basins. In total, 0.9% of the Orinoco's average annual flow at its ocean mouth is diverted by the Casiquiare Channel to the Amazon. Through this channel, the Upper Orinoco River contributes 0.16% of the Amazon's average annual flow at its mouth.

**Author Contributions:** Conceptualization: A.L., J.L.L. and P.G.; Methodology: A.L., J.L.L. and S.Y.; Software: S.Y.; Validation: A.L. and P.G.; Formal analysis: A.L., J.L.L. and S.Y.; Investigation: A.L., J.L.L. and S.Y.; Data curation: A.L., S.Y. and P.G.; Writing—original draft preparation: A.L., J.L.L., P.G. and S.Y.; Writing—review and editing: A.L., J.L.L. and S.Y.; Project administration: A.L. and P.G.; Funding acquisition: P.G.

**Funding:** This research received no external funding.

**Acknowledgments:** This study has been carried out under the sponsorship of the Simón Bolívar University (USB, Venezuela) and the University of Brasília (UnB, Brazil) with the assistance of the Venezuelan and Brazilian armed forces and the Institute of Research for Development (IRD, France), during the international fluvial expedition 'Humboldt-Amazonia 2000'.

**Conflicts of Interest:** The authors declare no conflict of interest.

## References

1. Laraque, A.; Castellanos, B.; Steiger, J.; Lopez, J.L.; Pandi, A.; Rodriguez, M.; Rosales, J.; Adèle, G.; Perez, J.; Lagane, C. A comparison of the suspended and dissolved matter dynamics of two large inter-tropical rivers draining into the Atlantic Ocean: The Congo and the Orinoco. *Hydrol. Process.* **2013**, *27*, 2153–2170.
[CrossRef]

2. Pérez, H.D. *Mediciones Hidrológicas en el Sector Alto Orinoco—Caño Casiquiare*; MARNR: Caracas, Venezuela, 2002.

3. The Global Runoff Data Centre GRDC—The World-Wide Repository of River Discharge Data and Associated Metadata. Available online: https://www.bafg.de/GRDC/EN/01_GRDC/grdcnode.html (accessed on 15 March 2019).

4. Sternberg O'Reilly, H. *The Amazon River of Brazil*; Geographische Zeitschrift, Beihefte Erdk. Wissen 40; Franz Steiner: Wiesbaden, Germany, 1975; p. XII+ 74.
5. Goodland, R.; Irwin, H. *A Selva Amazônica: Do Inferno Verde ao Deserto Vermelho*; Universidade de São Paulo: São Paulo, Brazil, 1976.

6. Von Humboldt, A. *Voyage Dans Les Régions Equinoxiales du Nouveau Continent*; Tome II; N. Maze: Paris, France, 1819; Available online: http://gallica.bnf.fr/ark:/12148/bpt6k61299w (accessed on 14 March 2018).

7. La Condamine, C.M. *Relation Abrégé D'un Voyage Fait Dans L'amérique Méridionale*; Académie des Sciences: Paris, France, 1881.

8. Georgescu, C.P. *Los Ríos de la Integración Suramericana*; USB: Caracas, Venezuela, 1984.

9. USACE (U.S. Army Corps of Engineers). Orinoco-Casiquiare-Negro Waterway. In *Venezuela-Colombia-Brasil*; USACE: Washington, DC, USA, 1943.

10. Georgescu, P.; Georgescu, C.P.; Laraque, A. *Le Casiquiare ... un Fleuve qui Relie Deux Fleuves! Bulletin de la Société de Géographie*; Société de Géographie: Paris, France, 2008.

11. Nordin, C.F.; Meade, R.H. The Amazon and the Orinoco. In *McGraw-Hill Yearbook of Science & Technology*; McGraw-Hill Inc.: New York, NY, USA, 1986; pp. 385–390.

12. Meade, R.H.; Koehnken, L. Distribution of the river dolphin, tonina "Inia geoffrensis", in the Orinoco River Basin of Venezuela and Colombia. *Interciencia* **1991**, *16*, 300–312.

13. Gessner, F. *Ensayo de una comparación química entre el Río Amazonas, el Río Negro y el Río Orinoco. Acta Científica Venez.* **1960**, *11*, 63–64.

14. Edwards, A.M.C.; Thornes, J.B. Observations on the dissolved solids of the Casiquiare and Upper Orinoco, April-June. *Amazoniana* **1968**, *2*, 245–256.

15. Edmond, J.; Palmer, M.; Measures, C.; Grant, B.; Stallard, R. The fluvial geochemistry and denudation rate of the Guayana Shield in Venezuela, Colombia and Brazil. *Geochim. Cosmochim. Acta* **1995**, *59*, 3301–3325. [CrossRef]

16. Sidder, G.B.; Mendoza, S.V. *Geology of the Venezuelan Guayana Shield and Its Relation to the Entire Guayana Shield*; USGS Open-File Rept; US Geological Survey: Reston, VA, USA, 1991; pp. 91–141.

17. Goldstein, R.J. *Angelfish. Barron's*; B.E.S. Publishing: Hauppauge, NY, USA, 2001; 96p.

18. Stern, K.M. Der Casiquiare-Kanal, einst und jetzt. *Amazoniana* **1970**, *2*, 401–416.

19. Vareschi, V. La Bifurcación del Orinoco: Observaciones Hidrográficas Ecológicas de la Expedición Conmemorativa de Humboldt del año 1958. *Acta Científica Venez.* **1963**, *14*, 9.

20. Besler, H. *Beobachtungen zum Casiquiare-problem* (Remarques sur le problème du Casiquiare). *Erdkunde* **1995**, *49*, 152–156. [CrossRef]

21. Grohmann, C.H. Tectonic Controls on the Capture of the Orinoco River and Formation of the Casiquiare Canal, Venezuela. *Geol. Soc. Am. Abstr. Programs* **2012**, *44*, 188.

22. Stokes, M.F.; Goldberg, S.L.; Taylor, J. Ongoing river capture in the Amazon. *Geophys. Res. Lett.* **2018**. [CrossRef]

23. Stern, K.M. La Génesis del Casiquiare. *Acta Científica Venez.* **1954**, *5*, 52–53.

24. Subramanya, K. *Flow in Open Channels*, 3rd ed.; Tata McGraw-Hill Education: New York, NY, USA, 1982.

25. USGS. *Shuttle Radar Topography Mission, 1 Arc Second Scene SRTM_u03_n008e004, Unfilled Unfinished 2.0, Global Land Cover Facility*; University of Maryland: College Park, MD, USA, 2000.

26. Roux, C.; Alber, A.; Bertrand, M.; Vaudor, L.; Piégay, H. "Fluvial Corridor": A new ArcGIS toolbox package for multiscale riverscape exploration. *Geomorphology* **2015**, *242*, 29–37. [CrossRef]

27. Ron, E.; León, R.; Mejía, A.; Colmenares, G. *Informe de Las Condiciones Hidráulicas del Caño Casiquiare*; Revista El Agua: Caracas, Venezuela, 1982.

28. MOP. *Mediciones en Ríos Grandes*; Ministerio de Obras Públicas, División de Hidrología: Caracas, Venezuela, 1972.

29. Vegas, V.; Paolini, J.; Herrera, R. A physico-chemical survey of black water rivers from the Orinoco and the Amazon basins in Venezuela. *Arch. Hydrobiol.* **1988**, *111*, 491–506.

30. Cordoba, J.R. Caracterización del Funcionamiento Hidrológico e Hidráulico-Fluvial del Delta del Río Orinoco. In *PDVSA-Informe Desarrollo Armónico de Oriente DAO*; FUNINDES, USB: Caracas, Venezuela, 1999.

31. Silva, G. La cuenca del río Orinoco: Visión hidrográfica y balance hídrico. The Orinoco River Basin: Hydrographic view and its hydrological balance. *Rev. Geogr. Venez.* **2005**, *46*, 75–108.

32. Laraque, A.; Pouyaud, B.; Rocchia, R.; Robin, R.; Chaffaut, I.; Moutsambote, J.M.; Maziezoula, B.; Censier, C.; Albouy, Y.; Elenga, H.; et al. Origin and function of a closed depression in equatorial humid zones: The lake Tele in the north Congo. *J. Hydrol.* **1998**, *207*, 236–253. [CrossRef]
33. Molinier, M.; Guyot, J.L.; Oliveira, E.; Guimarães, V. *Les Régimes Hydrologiques de L'amazone et de Ses Affluents*; IAHS: Paris, France, 1996.

34. Palacios, M.O. *El Orinoco, Tercer río Del Mundo*; Fundación Avensa y Fundación Terramar: Caracas, Venezuela, 1998.

35. Carreteiro, R.P. *A Navegação na Amazônia*; Editora Calderaro Ltda.: Manaus, Brazil, 1987; pp. 62–65.

36. CAF. *Los Ríos Nos Unen. Integración Fluvial Suramericana*; Corporación Andina de Fomento: Caracas, Venezuela, 1998.

37. Laraque, A.; Georgescu, P.; Hernández, D.P. El canal del Casiquiare: Una singularidad hidrológica única en el mundo. In *Proceedings of the XXII Congreso Latinoamericano de Hidráulica, Ciudad Guayana, Venezuela, 9-12 October 2006*.

38. Lewis, W.M.; Weibezahn, F.H. The chemistry and phytoplankton of the Orinoco and Caroni rivers, Venezuela. *Arch. Hydrobiol.* **1981**, *91*, 521–528.

39. Lewis, W.M.; Saunders, J.F. Concentration and transport of dissolved and suspended substances in the Orinoco River. *Biogeochemistry* **1989**, *7*, 203–240. [CrossRef]

40. Paolini, J. Organic carbon in the Orinoco River (Venezuela). *Verh. Internat. Verein. Limnol.* **1991**, *24*, 2077–2079. [CrossRef]

41. Brown, E.; Edmond, J.; Raisbeck, G.; Bourlès, D.; Yiou, F.; Measures, C. Beryllium isotope geochemistry in tropical river basins. *Geochim. Cosmochim. Acta* **1992**, *57*, 1607–1624. [CrossRef]

42. Winemiller, K.; Lopez-Fernandez, H.; Taphorn, D.C.; Nico, L.G.; Duque, A.B. Fish assemblages of the Casiquiare River, a corridor and zoogeographical filter for dispersal between the Orinoco and Amazon basins. *J. Biogeogr.* **2008**, *35*, 1551–1563. [CrossRef]

43. Carretier, S.; Tolorza, V.; Regard, V.; Aguilar, G.; Bermúdez, M.A.; Martinod, J.; Guyot, J.L.; Hérail, G.; Riquelme, R. Review of erosion dynamics along the major NS climatic gradient in Chile and perspectives. *Geomorphology* **2018**, *300*, 45–68. [CrossRef]

44. Weibezahn, F.H.; Alvarez, H.; Lewis, W.M., Jr. (Eds.) *The Orinoco River as an Ecosystem*; EDELCA: Caracas, Venezuela, 1990.

45. Winemiller, K.; Willis, S.C. The Vaupes Arch and Casiquiare Canal: Barriers and Passages. In *Historical Biogeography of Neotropical Freshwater Fishes*; Albert, J.S., Reis, R., Eds.; The Regents of the University of California: Oakland, CA, USA, 2011.

46. McLeod, S.L. *The Evolution of the Echimamish River: Northern Manitoba*; FGS—Electronic Theses & Dissertations (Public): Winnipeg, MB, Canada, 1975; Available online: http://hdl.handle.net/1993/6165 (accessed on 7 August 2019).

47. Gumbricht, T.; McCarthy, T.S.; Merry, C.L. The topography of the Okavango Delta, Botswana, and its tectonic and sedimentological implications. *S. Afr. J. Geol.* **2001**, *104*, 243–264. [CrossRef]

48. Siergieiev, D. Impact of Hydropower Regulation on River Water Geochemistry and Hyporheic Exchange. Ph.D. Thesis, Luleå Tekniska Universitet, Luleå, Sweden, 2013.

<figure><img src="image_212.png" /></figure>

© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
<figure><img src="image_213.png" /></figure>
Article

# Inverse Estuaries in West Africa: Evidence of the Rainfall Recovery?

Luc Descroix <fnref n="1" />,<fnref n="2" />,*, Yancouba Sané <fnref n="2" />,<fnref n="3" />, Mamadou Thior <fnref n="2" />,<fnref n="3" />, Sylvie-Paméla Manga <fnref n="2" />,<fnref n="3" />,<fnref n="4" />,
Boubacar Demba Ba <fnref n="2" />,<fnref n="3" />, Joseph Mingou <fnref n="2" />,<fnref n="3" />, Victor Mendy <fnref n="3" />,<fnref n="5" />, Saloum Coly <fnref n="3" />,<fnref n="5" />, Arame Dièye <fnref n="2" />,<fnref n="3" />,
Alexandre Badiane <fnref n="1" />,<fnref n="2" />,<fnref n="3" />, Marie-Jeanne Senghor <fnref n="2" />, Ange-Bouramanding Diedhiou <fnref n="2" />, Djiby Sow <fnref n="2" />,<fnref n="3" />,
Yasmin Bouaita <fnref n="2" />, Safietou Soumaré <fnref n="2" />,<fnref n="6" />,<fnref n="7" />, Awa Diop <fnref n="2" />,<fnref n="8" />, Bakary Faty <fnref n="9" />, Bamol Ali Sow <fnref n="3" />,<fnref n="5" />,
Eric Machu <fnref n="5" />,<fnref n="10" />, Jean-Pierre Montoroi <fnref n="11" />, Julien Andrieu <fnref n="2" />,<fnref n="7" /> and Jean-Pierre Vandervaere <fnref n="12" />

Received: 26 January 2020; Accepted: 24 February 2020; Published: 28 February 2020

**Abstract:** In West Africa, as in many other estuaries, enormous volumes of marine water are entering the continent. Fresh water discharge is very low, and it is commonly strongly linked to rainfall level. Some of these estuaries are inverse estuaries. During the Great Sahelian Drought (1968–1993), their hyperhaline feature was exacerbated. This paper aims to describe the evolution of the two main West African inverse estuaries, those of the Saloum River and the Casamance River, since the end of the drought. Water salinity measurements were carried out over three to five years according to the sites in order to document this evolution and to compare data with the historical ones collected during the long dry period at the end of 20th century. The results show that in both estuaries, the mean water salinity values have markedly decreased since the end of the drought. However, the Saloum estuary remains a totally inverse estuary, while for the Casamance River, the estuarine turbidity maximum (ETM) is the location of the salinity maximum, and it moves according to the seasons from a location 1–10 km downwards from the upstream estuary entry, during the dry season, to a location 40–70 km downwards from this point, during the rainy season. These observations fit with the functioning of the mangrove, the West African mangrove being among the few in the world that are markedly increasing since the beginning of the 1990s and the end of the dry period, as mangrove growth is favored by the relative salinity reduction. Finally, one of the inverse estuary behavior factors is the low fresh water incoming from the continent. The small area of the Casamance and Saloum basins

<footnote n="1">¹ IRD UMR PALOC MNHN/IRD/Sorbonne-Université, 75231 Paris, France; a.badiane785@zig.univ.sn</footnote>

<footnote n="2">² LMI PATEO, UGB, BP 234 Saint Louis, Senegal; saneyancouba@gmail.com (Y.S.); thioryaz@yahoo.fr (M.T.); s.manga4555@zig.univ.sn (S.-P.M.); badembaba@gmail.com (B.D.B.); josephmingou30@gmail.com (J.M.); dieyearame91@gmail.com (A.D.); mjsenghor@gmail.com (M.-J.S.); bouramanding@gmail.com (A.-B.D.); sowsowdjiby@gmail.com (D.S.); yasmin.bouaita@gmail.com (Y.B.); safisoumare1@gmail.com (S.S.); a.diop754@zig.univ.sn (A.D.); Julien.ANDRIEU@univ-cotedazur.fr (J.A.)</footnote>

<footnote n="3">³ UASZ Université Assane Seck de Ziguinchor, BP 523 Ziguinchor, Senegal; victormendy97@yahoo.fr (V.M.); colysaloum@gmail.com (S.C.); bsow@univ-zig.sn (B.A.S.)</footnote>

<footnote n="4">⁴ Université de Lorraine, UFR des Sciences Humaines et Sociales, 54015 Nancy, France</footnote>

<footnote n="5">⁵ LMI ECLAIRS, UASZ, ENS, BP 5036 Dakar, Senegal; eric.machu@ird.fr</footnote>

<footnote n="6">⁶ LaSTEE, Ecole Polytechnique de Thiès, DPA 10 Thiès, Senegal</footnote>

<footnote n="7">⁷ ESPACE Lab, Université Côte d'Azur, UFR Espaces & Cultures Campus, 06204 Nice, France</footnote>

<footnote n="8">⁸ Université Versailles St Quentin en Yvelines, UFR Sciences Sociales, 78280 Guyancourt, France</footnote>

<footnote n="9">⁹ DGPRE, Direction de la Gestion et la Planification des Ressources en Eau, section 2, 20000 Diamniadio, Senegal; bakaryfaty@gmail.com</footnote>

<footnote n="10">¹⁰ IRD/Laboratoire d'Océanographie Physique et Spatiale (LOPS), IUEM, Univ. Brest, CNRS, IRD, Ifremer, 29280 Plouzané, France</footnote>

<footnote n="11">¹¹ IRD/IEES; Institut d'écologie et des sciences de l'environnement, 93143 Bondy, France; jean-pierre.montoroi@ird.fr</footnote>

<footnote n="12">¹² IGE/Université Grenoble Alpes; Institut des Géosciences et Environnement, 38058 Grenoble, France; jean-pierre.vandervaere@univ-grenoble-alpes.fr</footnote>

<footnote>* Correspondence: luc.descroix@ird.fr</footnote>
(20,150 and 26,500 km² respectively) is to be compared with the basins of their two main neighbor basins, the Gambia River and the Senegal River, which provide significant fresh water discharge to their estuary.

**Keywords:** water salinity; inverse estuaries; West Africa; drought; mangrove

# 1. Problematics, State of the Art

West African Sahelian and Sudanian areas commonly have very flat coast plains. Therefore, rivers have long estuaries in front of their low hydraulicity. The Gambia River estuary, 450 km long, is considered the second longest in the world after that of the Amazon River, although its basin has an area 100 times and a discharge 1000 times smaller. In West Africa, only great basins provide enough fresh water to reach pushing saline waters seasonally out of the river mouths. In this constraining context, it is very easy for marine salt water to enter profoundly in the continent, and it is also easy for the tide to influence deeply the river water level within the continent. An inverse estuary is an estuary in which freshwater input is less than the losses due to evaporation; such estuaries contain hyperhaline water (e.g., Laguna Madra in Texas; [1]). Inverse estuaries were also defined by Pritchard [2] as the ones where salinity increased with distance to the mouth. In an inverse estuary, sea water enters the estuary from downstream to upstream to compensate for the losses due to evaporation, carrying salt, which raises concentration [3].

Some of these estuaries have very low fresh water income and are located in areas with very high evaporation. This leads to an increase in water density and a hypersaline density downwelling. Therefore, a surficial water flow is noticed upwards, and another flow is noticed near the bottom downwards [4]. In the case of the Saloum River, the increase is observed to be in a roughly linear fashion with distance to the sea [5].

These inverse estuaries are commonly found in semiarid or arid areas, such as southern Australia [6], northern Australia [7], some areas of Texas [1], Baja California (NW Mexico) [8,9], and in the Sahel [10], among others. Such hypersaline estuaries are relatively rare on the Earth. Other examples are the Red Sea and the Persian Gulf in the northern Indian Ocean [11], Shark Bay and Exmouth Gulf in Western Australia [12], and the hypersaline Coorong estuary/lagoon in South Australia [13,14]. The characteristics of Spencer Gulf, South Australia, are that evaporation exceeds precipitation all year round and that the spring-neap tidal cycle is greatly exaggerated [6].

In West Africa, as in the other estuaries, enormous volumes of marine water are entering the continent [15]. Fresh water discharge is commonly strongly linked to rainfall level [5]. During the Great Sahelian Drought (1968–1993), some main rivers completely dried up; this was the case of the Niger River in Niamey in May 1985 [16]. In extreme cases, “a negative water budget has even more drastic effects on smaller rivers: discharge becomes negative, (and) seawater may invade the estuary which becomes hyperhaline” [17]. As an example, “the Casamance River estuary, in a dry year, and during the dry season, can be changed into an evaporation basin, concentrating marine salts coming from [the] Atlantic Ocean and becoming a threat to fluvio-marine areas soils” [17].

Southward Dakar Senegambian estuaries are subject to [this] “unusual hydrodynamical regime caused by weak or absent run-off” [18]. Such a process has been occurring in two coastal “rivers” of Senegal, the Casamance and the Saloum Rivers, which are both actually tide-influenced “inverse estuaries” [5]. Therefore, the West African Sudano-Sahelian coast is one of the regions in the world where inverse estuaries are observed [19].

West Africa has shown a great interdecadal rainfall variability. Few data before 1950 allow highlighting two dry periods in 1910–1915 and 1940–1944. After the Second World War, the number of rain gauges increased significantly, and the following evolution can be described [20–22]:
- a very rainy period from 1950 to 1967

- a long and very dry period from 1968 to 1993 (whole West Africa) and from 1968 to 1998 in Senegambia and Mauritania

- a rainfall recovered period (1994 to 2018 in WA, 1999 to 2018 in Senegambia), which was characterized by a rainfall annual amount close to the 1918–2017 average and an intensification of rainfall: an increase in rainfall intensity [23] and in the number of extreme rainfall events [24] are observed.

The evolution in coastal Senegal is similar to the regional one. Figure 1 shows the location of Senegambia (Figure 1a) and the location of rain gauge stations (Figure 1b). Figure 2 gives the rainfall evolution at different spatiotemporal scales. Figure 2a shows the evolution at three Senegalese coastal stations from 1917 to 2017, and Figure 2b shows the one in the inverse estuaries areas, southward from Dakar, from 1950 to 2017. Figure 2c,d give the SPI (Standard Precipitation Index) respectively for Senegambia and for the Casamance River basin. This confirms also the conclusions of Faye and Sané (2015; [25]), who observed the end of the long dry period in 1996 for the Casamance River Basin.

<figure><img src="image_214.png" /><figcaption>Figure 1. Location of Senegambia (a) and location of main rivers, boundaries, and rain gauge stations (b).</figcaption></figure>

<figure><img src="image_215.png" /><figcaption>Figure 2. *Cont.*</figcaption></figure>
<figure><img src="image_216.png" /><figcaption>(b)</figcaption></figure>

<figure><img src="image_217.png" /><figcaption>(c)</figcaption></figure>

<figure><img src="image_218.png" /><figcaption>(d)</figcaption></figure>

**Figure 2.** (a) Evolution of annual rainfall from 1917 to 2017 in three coastal Senegalese stations; (b) evolution of annual rainfall in the inverse estuary area; Rainfall Index Evolution in Senegambia (SPI, Standard Precipitation Index) (c), and in Casamance River basin: Ziguinchor, Kolda, Velingara (SPI Standard Precipitation Index) (d).
Teleconnections with the ITCZ (Inter Tropical Convergence Zone) fluctuations and their impact on flooding were analyzed by Mahé et al. (2013) [26].

When a river bed is close to the sea level, sea water may ingress during the low-water seasons. If the freshwater inflow is less than the loss through evaporation, salinity becomes higher than that in the sea. During the drought (1968–1998) Binet et al. 1995 [27] wrote, “This happens in the Senegal and the Casamance, particularly during the “current” drought”.

Before 1960, marine water intrusions into the Senegal River during the dry season were negligible; but, in 1969 they reached 150 km upstream, and in 1978, they reached even 200 km [28]. Before the construction of the Diama (1986) and Manantali (1988) dams, high salinity values were observed especially during the dry period (1968–1986), particularly in the later years when salinity was concentrated due to a succession of very dry years in the second paxorysm of the Great Drought (1982–1986). Values above 35‰ were almost each year noticed at the end of the dry season (June and July) 80 km upstream from the mouth by Saos et al. (1984) [29]. These authors evidenced the inverse functioning of the estuary. Such behavior is widespread in the other Senegambian estuaries (Sine-Saloum, Gambia, and Casamance Rivers); however, it was strictly exceptional and of very short duration in the Senegal estuary [28]. These authors observed that the recovery of normal functioning is quite long (27 days in 1981 and 38 days in 1982).

Similar to the Casamance and Saloum estuaries, the semidiurnal microtidal Somone River estuary (80 km southeast of Dakar), where the maximum tidal range is about two meters, is characterized by an inverse salinity gradient [30,31]. Fluvial flows in the Somone estuary are null. Therefore, the salinity gradient is inverse; the only fresh water incomes observed during the 2007–2010 period are provided by rainfall and groundwater [31].

Hydrochemical analysis confirms that the Casamance River and Saloum River are “inverse estuaries”, in which the water is salted during most of the year (at least 9 months) and hypersaline at the end of dry season [32].

Some estuaries within dry tropical areas (e.g., in Australia) show a combination of estuarian “normal” and “inverse” modes [4]. Inverse functioning of the Saloum River estuary, where salinity can reach 130 g/L [33], strongly affects fish and all marine species populations.

To conclude, Baran (1994) [34] summarized the context explaining that “Gambia river has a normal estuary, i.e., with decreasing salinity upwards. The Casamance estuary is inverse (rising salinity upwards) during the dry season and normal during the rainy season, while the Sine-Saloum is a “ria” where fluvial flows are null and the estuary is always inverse”.

This study proposes to describe the current behavior of two West African inverse estuaries and to compare it with that observed during the Sahelian dry period of the end of the 20th century.

## 2. Methodology

In order to document the current functioning of the Saloum and the Casamance estuaries (see location in Figure 3), two measurement devices were implemented:

- a twice a year direct measuring campaign through the Saloum and the Casamance estuaries since the end of 2016;

This measuring campaign included a network of measurement sites in both Saloum and Casamance estuaries, salinity measures with a refractometer PCE© 0100 (67250 Soultz-sous-forêt, France) and, for values lower than 20 mS/cm, a conductimeter HANNA© HI 98130 (Woonsocket, RI, the USA), which also gives temperature and pH. This campaign was carried out at the end of the dry season (may) and at the end of the rainy season (November);

- a settled ensemble of five multi-sensor devices localized in the Casamance River estuary only, since Jan 2014;
These devices are CTD sensors (Conductivity, Temperature, Depth) model Decagon© CTD 10 (Pullman, WA, the USA), each one was coupled with a Decagon© EM50 data logger.

According to Noblet (2012) [35], salinity is calculated as follows:

$$S_a = (0.72 \times \sigma - 3.06) \times (1 + 0.02 (T - 25)) \quad (1)$$

where:

- S<sub>a</sub> is salinity in psu,

- σ is conductivity (mS/cm).

- T is temperature in °C.

Equation (2) was validated with the measured values with both a refractometer and field conductometer. Since the bolons' water temperature always ranged between 21 and 28 °C, the deviation between measured and calculated values was low, rarely exceeding 2.5% (the highest observed difference was 4.8%).

Table 1 summarizes the data collected thanks to the implemented instruments.

<table><caption>Table 1. Data collected with Conductivity, Temperature, Depth (CTD) sensors around the Casamance estuary.</caption><thead><tr><th>Site *</th><th>Number of Measurements</th><th>Number of Averaged Points</th><th>% Missing Data</th><th>% Corrected Data</th></tr></thead><tbody><tr><td>Karabane</td><td>129,500</td><td>804</td><td>50</td><td>10</td></tr><tr><td>Ziguinchor</td><td>127,400</td><td>873</td><td>50</td><td>12</td></tr><tr><td>Goudomp</td><td>178,200</td><td>1653</td><td>20</td><td>10</td></tr><tr><td>Baila</td><td>214,700</td><td>2228</td><td>5</td><td></td></tr><tr><td>Niambalang</td><td>168,000</td><td>1547</td><td>15</td><td>8</td></tr></tbody></table>

* see location in Figure 3.

<figure><img src="image_219.png" /><figcaption>**Figure 3.** Location of cited West African estuaries. At **left**: geoclimatical areas (some annual rainfall amounts are indicated) and the main rivers network; at **right**: zoom on the study area at the local scale (dotted lines are the countries boundaries).</figcaption></figure>

CTD data were regularly calibrated with that of the conductimeters (up to 20 mS/cm) and that of the refractometer (from 5 g/L, i.e., approximately 3 mS/cm, up to 100 mS/cm).

- Refractometers were calibrated with distilled water at the beginning and at the end of each measurement fraction of the day.
- Conductimeters were calibrated with standard dilution products supplied by the provider at the beginning and at the end of each measurement fraction of day in order to ensure the quality of measured data.

### 3. Results

Figure 4a (Saloum estuary) and 4b (Casamance estuary) give the location of the observations and implementation devices.

<figure><img src="image_220.png" /><figcaption>Figure 4. Location of devices and measurement points: (a) Saloum; (b) Casamance.</figcaption></figure>
Despite the different periods of observation in each one of the settled devices, some general
observations can be made about the current behavior of the salinity in the estuaries.

3.1. *Casamance Estuary*

Figure 5a–e show the seasonal evolution of water salinity in five points of the Casamance estuary from 2013 to 2019. The locations of the five salinity measurement stations are shown in Figure 4b.

- The salinity annual variation increases from the mouth (located 2 km downstream from Karabane station) to the upstream part of the estuary.

○ this variation increases in the main reach of the Casamance river, from Karabane (Figure 5a) to Ziguinchor (70 km from the mouth; Figure 5b) and then to Goudomp (125 km from the mouth, Figure 5c)

○ it also increases from the main branch to secondary branches, the "tributaries" coming from the North, at Baila on the "Baila bolon" (Figure 5d; *bolon* is the mandinka name given to the saline rivers of the mangroves in West Africa), and from the South, at Niambalang on the Kamobeul bolon (Figure 5e);

- The mean salinity values remain close to those of the sea (slightly above, at 40 g/L instead of 35 g/L) in the estuary (at least until Goudomp), as well as in the south branch of Kamobeul Bolon; it is significantly higher in the northern branch of the Baila Bolon (55 g/L).

Figure 6a–f shows the spatial variation of salinity at two stages of the year in the Casamance estuary.
Figure 6a,c,e gives the salinity at the end of the rainy season in 2016, 2017, and 2018, respectively;
Figure 6b,d,f gives the salinity at the end of the dry season, in 2017, 2018, and 2019, respectively.

The following characteristics are observed:

- There is along all the year a fresh water income at the upstream entry of the main branch of the Casamance estuary (at Diopcounda Bridge); the same observation is made at the upstream origin of its main tributary, the Soungrougrou (Diaroumé Bridge); however, fresh water discharge is significantly lower in this river;

- There is always an estuarine turbidity maximum (ETM, [36,37]) in the upper part of both the Soungrougrou and the Casamance;

○ This area moves upstream during the dry season and it reaches the highest salinity values of the main reach of the Casamance (70 g/L in the Casamance, 100 g/L in the Soungrougrou);

○ It moves downstream during the rainy season, pushed by the fresh water discharge coming from the (small) basin of the Casamance and Soungrougrou rivers. The salinity values decrease during this period;

○ Downstream of this moving peak, salinity decreases all year long; then, Casamance river has an inverse estuary, however, its upper part has a normal functioning during a few kilometers in the dry season and over some tens of kilometers in rainy season;

○ In the main branch (Casamance), a second salinity peak is observed during some seasons at the confluence with the Soungrougrou river, due to the upper salinity values of the latter;

○ As observed in Figure 4a–f, salinity values are lower in the rainy season and higher in the dry season in the tributary bolons than in the main reach of the Casamance river estuary.
<figure><img src="image_221.png" /></figure>

<figure><img src="image_222.png" /><figcaption>Figure 5. *Cont.*</figcaption></figure>
<figure><img src="image_223.png" /></figure>

<figure><img src="image_224.png" /><figcaption>Figure 5. *Cont.*</figcaption></figure>
<figure><img src="image_225.png" /></figure>

**Figure 5.** Evolution of salinity (g.L⁻¹) at the recording CTD stations: (a) Karabane (February 2017–August 2019); (b) Ziguinchor (September 2015–August 2019); (c) Goudoump (February 2015–October 2018); (d) Baila (December 2013–August 2018); (e) Niambalang (June 2016–August 2019). Red points are reconstructed data based on tens of water salinity measurement made by a refractometer.

<figure><img src="image_226.png" /></figure>

**Figure 6.** *Cont.*
<figure><img src="image_227.png" /><figcaption>Figure 6. *Cont.*</figcaption></figure>
<figure><img src="image_228.png" /><figcaption>Figure 6. *Cont.*</figcaption></figure>
<figure><img src="image_229.png" /><figcaption>**Figure 6.** Casamance estuary salinity evolution: (a,c,e) at the end of the rainy season. (a) December 2016. (c) December 2017. (e). October 2018. (b,d,f), at the end of the dry season: (b) May 2017. (d) May 2018. (f) April 2019.</figcaption></figure>

## 3.2. Saloum Estuary

Figure 7a–f documents the salinity of the Saloum estuary at the end of the rainy season (Figure 7a,c,e for 2016, 2017, and 2018, respectively) and at the end of the dry season (Figure 7b,d,f for 2017, 2018, and 2019 respectively). The functioning of this estuary is simpler than that of the Casamance.

- The fresh water discharge in the rainy season is quasi null and thus completely negligible;

- Values are lower during the rainy season due to lower evaporation, rain fallen within the wide estuary zone, and the sum of many small inputs by surficial runoff and small bolons;

- The estuary has an inverse behavior all year long;

- The salinity always increases upwards; the maximal values are always measured completely upstream, at Kaolack bridge in the Saloum and at Fatick Bridge in the Sine river;

- The salinity is higher in the north branch of Saloum estuary than that in the mid one (Diomboss) and overall than that in the southern one (Bandiala) (see location Figure 4a);

- The Bandiala bolon is provided in fresh water by the Nema Bah river, which is a small permanent fresh water river; water comes from the abundant water table of the southern Saloum plateau.

The temporal interannual variability is discussed in the last part of the discussion.
<figure><img src="image_230.png" /><figcaption>Figure 7. *Cont.*</figcaption></figure>
<figure><img src="image_231.png" /><figcaption>Figure 7. *Cont.*</figcaption></figure>
<figure><img src="image_232.png" /><figcaption>**Figure 7.** Saloum estuary salinity evolution: (a,c,e), at the end of the rainy season. (a) September 2016. (c) Jan 2018. (e) November 2018. (b,d,f), at the end of the dry season: (b) June 2017. (d) May 2018. (f) April 2019.</figcaption></figure>
# 4. Discussion

## 4.1. A Comparison with Historical Data

One of the main references of past salinization (during the dry period 1968–1993) is the documentation about the Baila Bolon [3,38,39]. These authors highlighted the hypersalinization of the Baila bolon at its peak in 1984, as well as within the whole Senegambian valley [29]. In the Baila bolon, the mean salinity value was 96.8 g/L in 1983–1984; in the following years, salinity decreased until reaching 45.4 g/L in 1988–1989. Figure 8 shows the evolution of salinity at Baila (Baila bolon) during two periods. Salinization is not irreversible, and the rainfall recovery after 1988 allowed a quasi-complete desalinization at the end of the rainy season at the upstream sites of the estuary. The groundwaters have been recharged, and they flow through the salted rivers. They recharge the aquifers, rising water tables and allowing the conservation of all its fresh water [3].

<figure><img src="image_233.png" /><figcaption>**Figure 8.** Salinity (g/L) measured at Baila during the Sahelian « great drought: 1978–1979 [32] and 1982–1989 [3].</figcaption></figure>

The salinity at Baila station during the current period (Figure 5d), although presenting high interannual variability, is closer to the level observed during the 1978–1979 and 1988–1989 periods [3,32] than that measured during the 1982–1987 period [3]. The latter corresponds to the driest period of the Great Drought, the end of the second peak of dry years, which therefore is the period when the cumulated rainfall deficit was the highest. The 1978–1979 years are the period with a temporal rainfall recovery between the two “peaks” of droughts (1972–1974 and 1982–1984); the 1988–1989 period is also considered as the beginning of the rainfall recovery after the drought.

Table 2 shows the evolution of salinity at different points of the Casamance estuary, during
two periods:

- 1978–1979 (in [32])

- 2016–2019 (our measurements)
<table><caption>Table 2. Salinity evolution at several places in Casamance estuary in 1977–1979 (Marius, 1985, p. 48, [32]) and in 2015–2019 periods (see location in Figure 4b).</caption><thead><tr><th>Date</th><th>Salinity in g.L<sup>-1</sup></th><th>January 1978</th><th>May 1978</th><th>November 1978</th><th>May 1979</th><th>November 1975</th><th>May 2016</th><th>November 2016</th><th>May 2017</th><th>November 2017</th><th>May 2018</th><th>November 2018</th><th>May 2019</th></tr></thead><tbody><tr><td>Baila</td><td></td><td>46</td><td>76</td><td>6</td><td>68</td><td>8</td><td>60</td><td>18</td><td>82</td><td>18</td><td>88</td><td>16</td><td>75</td></tr><tr><td>Pointe St Georges</td><td></td><td>44</td><td>48</td><td>30</td><td>56</td><td>/</td><td>/</td><td>/</td><td>41</td><td>35</td><td>40</td><td>31</td><td>43</td></tr><tr><td>Etomé</td><td></td><td>41</td><td>93</td><td>4</td><td>91</td><td>3</td><td>88</td><td>5</td><td>99</td><td>15</td><td>90</td><td>8</td><td>100</td></tr><tr><td>Guidel</td><td></td><td>53</td><td>89</td><td>0</td><td>88</td><td>/</td><td>50</td><td>16</td><td>65</td><td>32</td><td>65</td><td>17</td><td>55</td></tr><tr><td>Marsassoum</td><td></td><td>42</td><td>68</td><td>20</td><td>66</td><td>/</td><td>/</td><td>35</td><td>65</td><td>45</td><td>65</td><td>35</td><td>62</td></tr><tr><td>Kandialo</td><td></td><td>16</td><td>60</td><td>0</td><td>52</td><td>/</td><td>/</td><td>31</td><td>75</td><td>59</td><td>87</td><td>43</td><td>101</td></tr><tr><td>Sedhiou</td><td></td><td>8</td><td>30</td><td>5</td><td>33</td><td>/</td><td>/</td><td>10</td><td>35</td><td>9</td><td>38</td><td>10</td><td>45</td></tr><tr><td>Diopcounda</td><td></td><td>1</td><td>14</td><td>0</td><td>15</td><td>/</td><td>/</td><td>4</td><td>21</td><td>2</td><td>25</td><td>0</td><td>11</td></tr><tr><td>Diaroumé</td><td></td><td>30</td><td>60</td><td>13</td><td>63</td><td>/</td><td>/</td><td>20</td><td>56</td><td>27</td><td>120</td><td>10</td><td>126</td></tr></tbody></table>
The comparison of the two periods allow noticing some differences:

- The minimal salinity values are more pronounced during the first period; however, this is partially due to the fact that in the second period, only two measurements per year are made;

- Salinity increases between the first and the second period in all the upper valleys (Baila Bolon at Baila, Soungrougrou at Diaroumé and Kandialo, and Casamance at Diopcounda);

- It decreases only at the Guidel station;

- It remains approximately equal in the mid basins (Sedhiou in the Casamance, Marsassoum in the Soungrougrou) and the lower valley (Etomé and Pointe St Georges).

This suggests that in spite of the rainfall recovery, the salinity did not decrease completely after the second "peak" of the drought, during the 1980s.

Figure 9 allows the same conclusion about the Saloum estuary; values of salinity are higher in the 2016–2019 period than during the 1980–1982 one. It is likely due to the strong increase in salinity caused by the second peak of the drought (1983–1985); the rainfall recovery since the 1990s was not sufficient to complete a true desalinization.

<figure><img src="image_234.png" /><figcaption>Figure 9. Salinity (g/L) vs. distance to mouth in the Saloum estuary, after [15].</figcaption></figure>

Debenay and Pagès (1987) [40] defined the Casamance estuary, and they noticed five areas to be distinguished from downstream to upstream:

- From 0 to 50 km: a marine domain, with salinity, tides, and behavior close to those of the sea;

- From 50 to 85 km: an intermediary area, with increasing salinity;

- From 85 to 175 km: an hyperhaline area where salinity can reach 100 g/L;

- From 175 to 225 km: an alternative domain where salinity can vary from 0 to 100 g/L between rainy and dry season and during a few weeks only;

- Upstream from 225 km: fresh water with low discharge of the continental area.
The rainfall recovery in Casamance [25] and in the whole Senegambia [21] did not modify this general functioning; however, it caused a significant decrease in salinity in both estuaries of the Saloum and Casamance rivers.

## 4.2. An Integrative Indicator: The Mangrove

The mangrove forest is located at the boundary between the ocean and continent. The global mangrove area is declining mainly due to human activities [41,42]. Inversely, it is worth noticing that in West Africa, after a decline period during the Great Drought of 1968–1993, the mangrove area was significantly rising [43–45]; it almost reached its ancient extension level within the Saloum estuary and it even exceeds it within the Casamance estuary. It is expanding quickly [44,46] due to rainfall recovery and overall due to sea level rise. An improvement of mangrove governance and reforestation (unfortunately, mainly proceeded with Rhizophora in places where Avicennia was most indicated) locally could have contributed to this extension (only 4% of the increase in the mangrove area is due to reforestation, with the other 96% being spontaneous in the Saloum estuary [46]; respectively, these figures are 7% and 93% in the Casamance estuary [45]).

The mangrove is very sensitive to salinity levels; it can resist a few weeks or months to very high or very low salinity; however, if these peaks are repeated each year or if salinity values remain very high for more than 7 or 8 months a year, mangrove would eventually die.

Gilman et al. (2008) [47] describe the role of sedimentation on the mangrove stability and the ways that sedimentation may impact mangrove resilience (such as sediment accretion and erosion, biotic contributions, belowground primary production, autocompaction, fluctuations in water table levels, and pore water storage) but conclude that there is no correlation between sedimentation rates and sea level rise (SLR).

Osland et al. (2018) [48] show how the accelerated SLR could favor sedimentation and biotic contributions and thus the extension upland or upriver of the mangrove forest. Mangrove forests in arid and semiarid climates are known to be particularly vulnerable to changes in rainfall and freshwater availability [48]; the SLR probably accelerated the desalinization of the West African bolons, and this is probably the main explaining factor of the current mangrove expansion.

As an example, and contrary to the conclusion of Dièye et al. (2013) [49], the natural opening of the "fleche de Sangomar" sand spit in 1987, at the northern side of the Saloum estuary, allowed the entry of important volumes of marine water (35 g/L) in a hyperhaline estuary; then, it provoked a reduction of the salinity in the inland estuary. The mangrove regeneration was firstly observed near Djiffere in 1992 (Mamadou Sow, personal communication), where the opening of the "fleche de Sangomar" allowed a strong relative desalinization and the mangrove recovery. Mangrove decline in the lower Saloum estuary after 1987 and the end of the drought (rainfall annual total amount began increasing after 1985) observed by Dièye et al. (2013) [49] must be due to factors other than salinization.

SLR seems to be influencing the mangrove extension by leading to a relative desalinization of inverse estuaries water and by helping greater volumes of sea water entering in estuaries with much higher salt rates. The rainfall recovery since the end of the 1980s made the Casamance estuary show normal behavior during an increasing proportion of the year; this recovery did not allow at the time the Saloum River to have periods with normal functioning. However, there is no evidence that Saloum was not yet, before the drought, a completely inverse estuary at least since the African Humid Period, before 4000 BP.

The current expansion of the mangrove is the result of the significant relative desalinization of the inverse West African estuaries.

## 4.3. Low Discharges Explaining the Inverse Estuaries

Saloum and Casamance rivers have very low discharge values due to geology (sedimentary originated mainly sandy soils), the topography is very flat and, in addition, in Casamance, vegetation is very dense and rice cropping is organized to store most of the rainwater during the rainy season. Table 3 indicates the runoff coefficient values to be compared with that of surrounding basins.
<table><caption>Table 3. Mean rainfall, discharge, and runoff coefficient (KE) is the inverse estuaries, compared with their surrounding main basins.</caption><thead><tr><th>River Basin</th><th>Area km<sup>2</sup></th><th>Annual Rain m<sup>3</sup></th><th>Annual Discharge m<sup>3</sup></th><th>% Total</th><th>Rain Depth mm</th><th>Mean Discharge m<sup>3</sup> s<sup>-1</sup></th><th>KE %</th></tr></thead><tbody><tr><td>Casamance at Diana Malari <sup>1</sup></td><td>4710</td><td>52,987 x 10<sup>6</sup></td><td>158,962 x 10<sup>3</sup></td><td>7.1</td><td>1125</td><td>5.08</td><td>3</td></tr><tr><td>Soungrougou <sup>1</sup></td><td>4480</td><td>51,609 x 10<sup>6</sup></td><td>51,610 x 10<sup>3</sup></td><td>2.3</td><td>1152</td><td>1.65</td><td>1</td></tr><tr><td>Mid Casamance basin <sup>1</sup></td><td>4150</td><td>54,158 x 10<sup>6</sup></td><td>216,630 x 10<sup>3</sup></td><td>9.7</td><td>1305</td><td>6.93</td><td>4</td></tr><tr><td>low Casamance basin Right bank <sup>1</sup></td><td>4323</td><td>56,631 x 10<sup>6</sup></td><td>283,156 x 10<sup>3</sup></td><td>12.7</td><td>1310</td><td>9.05</td><td>5</td></tr><tr><td>low Casamance basin Left bank <sup>1</sup></td><td>1560</td><td>23,587 x 10<sup>6</sup></td><td>235,872 x 10<sup>3</sup></td><td>10.6</td><td>1512</td><td>7.54</td><td>10</td></tr><tr><td>water body <sup>2</sup></td><td>927</td><td>12,857 x 10<sup>6</sup></td><td>1,285,749 x 10<sup>3</sup></td><td>57.6</td><td>1387</td><td>41.1</td><td>100</td></tr><tr><td>CASAMANCE BASIN</td><td>20,150</td><td>251,875 x 10<sup>6</sup></td><td>2,231,978 x 10<sup>3</sup></td><td>100</td><td>1250</td><td>71.4</td><td>8.9</td></tr><tr><td>Nema Bah <sup>3</sup></td><td>50</td><td>318 x 10<sup>6</sup></td><td>2365 x 10<sup>3</sup></td><td>0.31</td><td>637</td><td>0.075</td><td>8.2</td></tr><tr><td>Medina Djikoye <sup>3</sup></td><td>300</td><td>2145 x 10<sup>6</sup></td><td>10,373 x 10<sup>3</sup></td><td>1.38</td><td>716</td><td>0.33</td><td>6</td></tr><tr><td>Car Car at Tataguine <sup>4</sup></td><td>1950</td><td>11,349 x 10<sup>6</sup></td><td>5674 x 10<sup>3</sup></td><td>0.75</td><td>582</td><td>0.18</td><td>0.5</td></tr><tr><td>Sine at Fatick <sup>4</sup></td><td>3600</td><td>21,348 x 10<sup>6</sup></td><td>12,809 x 10<sup>3</sup></td><td>1.7</td><td>593</td><td>0.41</td><td>0.6</td></tr><tr><td>Saloum at Kaolack <sup>4</sup></td><td>9502</td><td>61,180 x 10<sup>6</sup></td><td>91,770 x 10<sup>3</sup></td><td>12.19</td><td>644</td><td>2.93</td><td>1.5</td></tr><tr><td>Lower basin <sup>4</sup></td><td>10,710</td><td>74,984 x 10<sup>6</sup></td><td>299,936 x 10<sup>3</sup></td><td>39.85</td><td>700</td><td>9.59</td><td>4</td></tr><tr><td>Water Body <sup>2</sup></td><td>388</td><td>3298 x 10<sup>6</sup></td><td>329,800 x 10<sup>3</sup></td><td>43.82</td><td>850</td><td>10.55</td><td>100</td></tr><tr><td>SALOUM BASIN</td><td>26,500</td><td>174,625 x 10<sup>6</sup></td><td>752,727 x 10<sup>3</sup></td><td>100</td><td>659</td><td>24.07</td><td>4.3</td></tr><tr><td>Senegal <sup>5</sup></td><td>337,000</td><td>2,527,500 x 10<sup>6</sup></td><td>20,183,040 x 10<sup>3</sup></td><td></td><td>751</td><td>640</td><td>8.0</td></tr><tr><td>Gambia <sup>5</sup></td><td>60,000</td><td>612,000 x 10<sup>6</sup></td><td>8,675,400 x 10<sup>3</sup></td><td></td><td>1020</td><td>275</td><td>14.2</td></tr><tr><td>Geba <sup>6</sup></td><td>12,440</td><td>164,208 x 10<sup>6</sup></td><td>1,970,496 x 10<sup>3</sup></td><td></td><td>1320</td><td>62.5</td><td>12</td></tr><tr><td>Corubal <sup>2</sup></td><td>26,000</td><td>445,640 x 10<sup>6</sup></td><td>10,249,200 x 10<sup>3</sup></td><td></td><td>1714</td><td>325</td><td>23</td></tr></tbody></table>

<footnote n="1">¹ Dacosta, 1989 [50]. ² calculated from GIS and rainfall data, + Guinea Bissau hydrological service for Geba River, ³ Mendy, 2010 [51]. ⁴ estimated from few existing data and double mass comparison with similar or surrounding basins, ⁵ [22]. ⁶ Sambou, 2019 [52]. See location of basins in Figure 3.</footnote>
Clearly, runoff coefficients are lower in the Saloum and Casamance basins compared with other basins that have a comparable total rainfall amount. This is one of the main explaining factors of their inverse functioning.

The Casamance River basin has a runoff coefficient just slightly higher than that of the Senegal River basin, although it receives 55% more rainwater than the latter. The runoff coefficient of the Saloum River basin is half of that of the Casamance River. The Geba River basin also has a low runoff coefficient compared with that of the other basins with similar rainfall amount.

These three basins have little runoff due to geological and topographical factors. In the two first cases, runoff is significantly enhanced by the rain water fallen directly in the estuary; this constitutes 44% of the total discharge of freshwater in the Saloum basin and 58% of that for the Casamance basin.

**5. Conclusions**

These observations about salinity spatiotemporal evolution carried out within two West African inverse estuaries allow us to attempt a few statements:

Firstly, about the functioning of estuaries, we can confirm that:

- The Saloum River estuary has a total inverse behavior, with salinity increasing upwards;

- The Casamance estuary has a spatially partial inverse functioning, with a point of maximum salinity migrating from 20–30 km of the upstream end estuary at the end of the dry season to 50–80 km downstream from this point at the end of the rainy season.

- Therefore, about the spatial variability, we observed:

- decreasing salinity downwards from the peak of the ETM in the Casamance estuary and from the upstream entry of the estuary at Kaolack in the Saloum river;

- Increasing salinity seasonal variability in the tributary bolongs;

- A similar behavior in the bolongs than in the Casamance upper estuary.

Finally, the salinity seasonal variability increases with the distance to the ocean.

A relative desalinization is attested by the comparison of measurement realized since 2013. Besides, the progressive post-drought rainfall recovery led to a decreasing bolongs water salinity but answering with a 10–15-year delay. The mangrove spectacular recovery is a good indicator of the progressive reduction of the bolongs’ water salinity.

However, the low discharge and runoff coefficient values in the continental part of the basins is one of the main and persistent explaining factors of this inverse functioning.

**Author Contributions:** Conceptualization, Y.S., M.T., S.-P.M., M.-J.S. and J.A.; Data curation, J.M., A.-B.D., D.S., Y.B., S.S. and A.D.(Awa Diop); Formal analysis, J.M., V.M., A.D. (Arame Diève), A.B., A.-B.D. and B.F.; Funding acquisition, Y.S., M.T., S.-P.M., B.D.B. and J.M.; Investigation, Y.S., M.T., S.-P.M., B.D.B., J.M., S.C., A.D. (Arame Diève), M.-J.S., D.S., S.S., A.D. (Awa Diop), B.F., B.A.S. and J.-P.V.; Methodology, V.M., S.C., A.D. (Arame Diève), A.B., M.-J.S., A.-B.D., S.S., A.D. (Awa Diop) and B.F.; Project administration, B.A.S.; Resources, B.D.B., V.M. and S.C.; Software, A.B.; Supervision, E.M. and J.-P.M.; Validation, J.A. and J.-P.V.; Visualization, D.S. and Y.B.; Writing—original draft, L.D. Y.S. and M.T.; Writing—review and editing, L.D. S.-P.M., B.A.S., E.M., J.-P.M., J.A. and J.-P.V. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by AFD (French Development Agency) (convention AFD CZZ 1894 01K).

**Acknowledgments:** The authors acknowledge the AFD (Agence Française de Développement) funded projet "Développement durable des zones littorales (Sénégal, Guinée Bissau, Guinée): vers une gouvernance citoyenne des territoires"(Sustainable development of coastal areas (Senegal, Guinea-Bissau, Guinea): towards a citizen governance of the territories (convention AFD CZZ 1894 01K), as well as the Grdr Migrations Citoyenneté Développement NGO for the project management.

**Conflicts of Interest:** The authors declare no conflict of interest.

**References**

1. Paturej, E. Estuaries, Types, Role and Impact on Human Life. 2008. Available online: https://www.researchgate.net/publication/228488312 (accessed on 11 September 2019).
2. Pritchard, D.W. What is an estuary: Physical viewpoint. In *Estuaries*; Lauff, G.H., Ed.; American Association for the Advancement of Science: Washington, DC, USA, 1967; Volume 83, pp. 3–5.

3. Saos, J.-L.; Thiebaux, J.-P. *Evolution de la salinité en Basse Casamance: Exemple du marigot de BAILA*; Equipe Pluri Disciplinaire d'Etude des Ecosystèmes Côtiers; Rapport Final; UNESCO Sea Sciences Division, PNUD, UCAD, UNESCO Breda Ed (the NL); Etude des Estuaires du Sénégal: Casamance, Sénégal, 1991.

4. Alvera-Azcarate, A. 2011. Available online: http://modb.oce.ulg.ac.be/mediawiki/upload/Aida/OCEA0011/3_ESTUARIES.pdf (accessed on 11 September 2019).

5. Pagès, J.; Citeau, J. Rainfall and salinity of a Sahelian estuary between 1927 and 1987. *J. Hydrol.* **1989**, *113*, 325–341. [CrossRef]

6. Nunes Vaz, R.A.; Lennon, G.W.; Bowers, D.G. Physical behaviour of a large, negative or inverse estuary. *Cont. Shelf Res.* **1990**, *10*, 277–304. [CrossRef]

7. Tomczak, M. *Examples of Estu Aries and Their Classification* 2000. Available online: https://incois.gov.in/Tutor/ShelfCoast/chapter13.html (accessed on 11 September 2019).

8. Lavin, M.F.; Godinez, V.M.; Alvarez, L.G. Inverse-estuarine features of the upper gulf of California. *Estuar. Coast. Shelf Sci.* **1998**, *47*, 769–795. [CrossRef]

9. De Gutierrez Velasco, G.; Winant, C.D. Wind- and Density-driven circulation in a well-mixed inverse estuary. *J. Phys. Oceanogr.* **2004**, *34*, 1103–1116. [CrossRef]

10. Villanueva, M.C. Contrasting tropical estuarine ecosystem functioning and stability: A comparative study. *Estuar. Coast. Shelf Sci.* March 2015, 155, 89–103. [CrossRef]

11. Kampf, J.; Sadrinasab, M. The circulation of the Persian Gulf: A numerical study. *Ocean Sci.* **2006**, *2*, 1–15. [CrossRef]

12. Edyvane, K.S. Australia, coastal ecology. In *Encyclopedia of Coastal Science*; Schwartz, M., Ed.; Springer: Dordrecht, The Netherlands, 2005; pp. 96–109.

13. Kampf, J.; Bell, D. The Murray/Coorong estuary: Meeting of the waters. In *Estuaries of the World*; Wolanski, E., Ed.; Springer: Dordrecht, The Netherlands, 2013; pp. 31–47.

14. Kampf, J. South Australia's Large Inverse Estuaries: On the Road to Ruin. In *Estuaries of Australia in 2050 and Beyond, Estuaries of the World*; Wolanski, E., Ed.; Springer Science & Business Media: Dordrecht, The Netherlands, 2014.

15. Barusseau, J.-P.; Diop, E.S.; Giresse, P.; Monteillet, J.; Saos, J.-L. Conséquences sédimentologiques de l'évolution climatique fini-Holocène (10²⁻¹⁰³ ans) dans le delta du Saloum (Sénégal). *Océanographie Trop. 1986*, *21*, 69–98.

16. Billon, B. Le Niger à Niamey: Décru et étiage 1985. *Cah. l'ORSTOM Série Hydrol.* **1985**, *21*, 3–22.

17. Boivin, P.; Le Brusq, J.-Y. Désertification et salinisation des terres au Sénégal. Problèmes et remèdes. In *Proceedings of the National Seminar on desertification, Dakar, Republic of Senegal*, 22–26 April 1985.

18. Barusseau, J.-P.; Diop, E.S.; Saos, J.-L. Evidence of dynamics reversal in tropical estuaries, geomorphological and sedimentological consequences (Salum and Casamance Rivers, Senegal). *Sedimentology* **1985**, *32*, 543–552. [CrossRef]

19. Thior, M.; Sy, A.A.; Diedhiou, S.O.; Cissé, I.; Gomis, J.S.; Descroix, L. Low Casamance inverse estuary: Impacts on water quality and agrosystems in islander area/Estuaire inverse de basse Casamance: Impacts sur la qualité de l'eau et des agrosystèmes en milieu insulaire. *Environ. Water Sci. Public Health Territ. Intell. J.* **2019**, *3*, 192–197.

20. Nicholson, S.E. The West African sahel: A review of recent studies on the rainfall regime and its interannual variability. *ISRN Meteorol.* **2013**, *2013*, 453521. [CrossRef]

21. Descroix, L.; Diongue Niang, A.; Panthou, G.; Bodian, A.; Sané, T.; Dacosta, H.; Malam Abdou, M.; Vandervaere, J.-P.; Quantin, G. Evolution Récente de la Mousson en Afrique de l'Ouest à Travers Deux Fenêtres (Sénégambie et Bassin du Niger Moyen). *Climatologie* **2015**, *12*, 25–43.

22. Descroix, L.; Guichard, F.; Grippa, M.; Lambert, L.A.; Panthou, G.; Gal, L.; Dardel, C.; Quantin, G.; Kergoat, L.; Bouaïta, Y.; et al. Evolution of surface hydrology in the Sahelo-Sudanian stripe: An updated synthesis. *Water* **2018**, *10*, 748. [CrossRef]

23. Panthou, G.; Lebel, T.; Vischel, T.; Quantin, G.; Sané, Y.; Ba, A.; Ndiaye, O.; Diongue-Niang, A.; Diop Kane, M. Rainfall intensification in tropical semi-arid regions: The Sahelian case. *Environ. Res. Lett.* **2018**, *13*, 064013. [CrossRef]
24. Panthou, G.; Vischel, T.; Lebel, T. Recent trends in the regime of extreme rainfall in the Central Sahel. *Int. J. Climatol.* **2014**, *34*, 3998–4006. [CrossRef]

25. Faye, C.; Sané, T. Le changement climatique dans le bassin-versant de la Casamance: Évolution et tendances du climat, impacts sur les ressources en eau et stratégies d'adaptation. In *Eaux et Sociétés Face au Changement Climatique Dans le Bassin de la Casamance*. Actes de l'Atelier Scientifique et Lancement de l'initiative Casamance: Un Réseau Scientifique au Service du Développement en Casamance. *Ziguinchor Sénégal* **2015**, 15–17, 280.

26. Mahé, G.; Lienou, G.; Descroix, L.; Bamba, L.; Paturel, J.-E.; Laraque, A.; Meddi, M.; Habaieb, M.; Adeaga, O.; Dieulin, C.; et al. The rivers of Africa: Witness of climate change and human impact on the environment. *Hydrol. Process.* **2013**, *27*, 2105–2114. [CrossRef]

27. Binet, D.; Le Reste, L.; Diouf, P.S. The influence of runoff and fluvial outflow on the ecosystems and living resources of West African coastal waters. In *Effects of Riverine Inputs on Coastal Ecosystems and Fisheries Resources*; Fisheries Technical Paper No.349; FAO—Food and Agriculture Organization of the United Nations: Rome, Italy, 1995; Volume 349, pp. 89–118. ISBN 978-9251036341.

28. Gac, J.-Y.; Kane, A. Le fleuve Sénégal: 2- flux continentaux de matières dissoutes à l'embouchure. *Sci. Géol. Bull.* **1986**, *39*, 151–172.

29. Saos, J.-L.; Kane, A.; Carn, M.; Gac, J.-Y. Persistance de la sécheresse au Sahel: Invasion marine exceptionnelle dans la vallée du fleuve Sénégal. In *Proceedings of the 10th meeting of Earth Sciences, Bordeaux, France*; 1984; p. 499.

30. Diop, E.S. La Côte Ouest Africaine du Saloum (Sénégal) à la Mellacorée (République de Guinée). Ph.D. Thesis, Université Louis Pasteur, Strasbourg, Paris, France, 1990.

31. Sakho, I. Evolution et Fonctionnement Hydro-Sédimentaire de la Lagune de la Somone, Petite Côte, Sénégal. Ph.D. Thesis, Université de Rouen, France and Université Cheikh Anta Diop de Dakar, Dakar, Sénégal, 2011.

32. Marius, C. Mangroves du Sénégal et de la Gambie. Ecologie, Pédologie, Géochimie, Mise en Valeur et Aménagement. Ph.D. Thesis, Louis Pasteur Strasbourg University, Strasbourg, Paris, 1985; 335p.

33. Diouf-Goudiaby, K. Influence de la salinité sur les déplacements et la croissance des juvéniles d'un poisson ubiquiste, Sarotherodon Melanotheron (Teleosteen, Cichlidae), dans les estuaires ouest-africains. Ph.D. Thèse, Université Montpellier 2, Montpellier, France, 2006; 176p.

34. Baran, E. Comparaison des ichtyofaunes estuariennes du Sénégal, de Gambie et de Guinée. In *Dynamiques et Usages de la Mangrove Dans les Pays des Rivieres du Sud (du Sénégal à la Sierra Leone) M-C*; Salem, C., Ed.; ORSTOM Editions, Paris-Bondy: Paris, France, 1994; pp. 84–92.

35. Noblet, J.-P. 2012. Available online: http://www.jf-noblet.fr/spe2012/2-eau/acti1-sel (accessed on 23 February 2020).

36. Toublanc, F.; Brenon, I.; Coulombier, T. Formation and structure of the turbidity maximum in the macrotidal Charente estuary (France): Influence of fluvial and tidal forcing. *Estuar. Coast. Shelf Sci.* **2016**, *169*, 1–14. [CrossRef]

37. Duy Vinh, V.; Ouillon, S.; Van Uu, D. Estuarine turbidity maxima and variations of aggregate parameters in the cam-nam trieu estuary, north vietnam, in early wet season. *Water* **2018**, *10*, 68. [CrossRef]

38. Olivry, J.-C.; Dacosta, H. *Le Marigot de Baila: Bilan des Apports Hydriques et Evolution de la Salinit*; Orstom: Dakar, Senegal, 1984; 150p. Available online: http://horizon/documentation.ird.fr/exl-doc/pleins_textes/divers11-07/17933.pdf#search=%22olivry%20dacosta%201984%20baila%22 (accessed on 21 September 2019).

39. Olivry, J.-C. Les conséquences durables de la sécheresse actuelle sur l'écoulement du fleuve Sénégal et l'hypersalinisation de la Basse-Casamance. In *Proceedings of the Influence of Climate Change and Climatic Variability on the Hydrologie Regime and Water Resources. In Proceedings of the Vancouver Symposium, Vancouver, BC, Canada, 9–22 August 1987*); IAHS Publisher: Wallingford, UK, 1987.

40. Debenay, J.-P.; Pagès, J. Foraminifères et thécamoebiens de l'estuaire hyperhalin du fleuve Casamance (Sénégal). *Revue d'Hydrobiologie Trop.* **1987**, *20*, 233–256.

41. Valiela, I.; Bowen, J.L.; York, J.K. Mangrove forests: One of the world's threatened major tropical environments. *Bioscience* **2001**, *51*, 807–815. [CrossRef]

42. FAO. Forest Resources Assessment working paper n°63. Forest Resources Division. In *Status and Trends in Mangrove Area Extent Worldwide*; Wilkie, M.L., Fortuna, S., Eds.; FAO: Rome, Italy, 2003. Available online: http://www.fao.org/docrep/007/j1533e/j1533e00.htm (accessed on 27 February 2020).
43. Andrieu, J. Dynamiques des Paysages Dans les Régions Septentrionales des Rivieres-du-Sud (Sénégal, Gambie, Guinée-Bissau). Ph.D. Thèse, Université dé Paris, Paris, France, 2008; 524.

44. Conchedda, G.; Durieux, L.; Mayaux, P. An object-based method for mapping and change analysis in mangrove ecosystems. *ISPRS J. Photogramm. Remote Sens.* **2008**, *63*, 578–589. [CrossRef]

45. Andrieu, J. L'évolution de la Mangrove (1979-2019) du Saloum au Géba, par Télédétection. In *Proceedings of the Communication to the International Colloque Vulnerability of Societies and Environment of Coastal and Estuarine West Africa Held on in Ziguinchor (Senegal)*, Dakar, Senegal, 19-22 November 2019.

46. Andrieu, J.; Lombard, F.; Fall, A.; Thior, M.; Ba, B.D.; Diémé, B.E.A. Botanical field-study and remote sensing to describe mangrove resilience in the Saloum Delta (Senegal) after 30 years of degradation narrative. *For. Ecol. Manag.* **2020**, *461*, 117963. [CrossRef]

47. Gilman, E.L.; Ellison, J.; Duke, N.C.; Field, C. Threats to mangroves from climate change and adaptation options: A review. *Aquat. Bot.* **2008**, *89*, 237–250. [CrossRef]

48. Osland, M.J.; Feher, L.C.; Lopez-Portillo, J.; Daya, R.H.; Suman, D.O.; Guzman Menendez, J.M.; Rivera-Monroy, V.H. Mangrove forests in a rapidly changing world: Global change impacts and conservation opportunities along the Gulf of Mexico coast. *Estuar. Coast. Shelf Sci.* **2018**, *214*, 120–140. [CrossRef]

49. Dièye, E.B.; Diaw, A.T.; Sané, T.; Ndour, N. Dynamique de la mangrove de l'estuaire du Saloum (Sénégal) entre 1972 et 2010. *Dynamics of the Saloum estuary mangrove (Senegal)* from 1972 to 2010. *Cybergeo. Eur. J. Geogr. Environ. Nat. Environ. Nat. Paysage* **2013**. [CrossRef]

50. Dacosta, H. Précipitations et Ecoulement Sur le Bassin de la Casamance. Ph.D. Thesis, University Cheikh Anta Diop, Dakar, Senegal, 1989; 283p.

51. Mendy, A. Ressources en Eau des Bassins Versants de la Nema et de Medina Djikoye. Ph.D. Thesis, University Cheikh Anta Diop, Dakar, Senegal, 2010; 335p.

52. Sambou, S. Contribution à la Connaissance des Ressources en Eau du Bassin Versant du Fleuve Kayanga/Géba (Guinée, Sénégal, Guinée-Bissau). Ph.D. Thesis, University Cheikh Anta Diop, Dakar, Senegal, 2019.

<figure><img src="image_235.png" /></figure>

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
MDPI

St. Alban-Anlage 66

4052 Basel

Switzerland

Tel. +41 61 683 77 34

Fax +41 61 302 89 18

www.mdpi.com

Water Editorial Office

E-mail: water@mdpi.com

www.mdpi.com/journal/water

<figure><img src="image_236.png" /></figure>
<figure><img src="image_237.png" /></figure>
MDPI

St. Alban-Anlage 66
4052 Basel
Switzerland

Tel: +41 61 683 77 34

Fax: +41 61 302 89 18

www.mdpi.com

<figure><img src="image_238.png" /></figure>

ISBN 978-3-0365-1237-2