A remote sensing-based classification approach for river mouths of the Amazon-influenced Guianas coast

The mouths of the rivers on the 1500-km-long Guianas coast, South America, are strongly influenced by large mud banks migrating alongshore from the Amazon River. To probe this influence, a river-mouth morphological classification scheme was carried out from satellite images, complemented by data from rare previous studies of estuarine hydrodynamics. The classification highlights a variety of morphological and dynamic river-mouth types that are further influenced by both fluvial water discharge and fluvial catchment rock type. Three basic types are identified: (1) river mouths diverted westward by capes and spits built from multi-decadal to multi-millennial accumulation of mud supplied by the Amazon; among these, mouths close to the Amazon are fixed by bedrock surrounded by mud; (2) small river mouths in French Guiana fixed by outcrops of Precambrian rocks that form headlands devoid of Amazon mud; and (3) large river mouths characterized by high water discharge that imparts an orientation normal to the coast; the largest two belonging to this type (the Essequibo in Guyana and the Maroni on the border between French Guiana and Suriname) exhibit infilled mouths that are transitioning towards deltas. These river mouths show a tropical seasonal estuarine circulation regime strongly influenced by Amazon mud. The influence of mud from the distant Amazon constitutes a unique aspect of the river mouths on the Guianas coast. Field and modelling studies in the future will be needed in order to achieve better characterization of the influence of Amazon mud on the estuarine circulation systems and of the distribution of sediment types within these river mouths, including sand supplied by the rivers.