Multi-decadal Changes on the Mud-dominated Coast of Western French Guiana: Implications for Mesoscale Shoreline Mobility, River-mouth Deflection, and Sediment Sorting

International audienceMud supplied by the Amazon River forms large banks that migrate along the Amazon-Orinoco Guianas coast under the influence of waves and currents, separated by 'inter-bank' zones (phases). Bank-welding onto the coast creates new land, followed by rapid mangrove colonization, and westward deflection of the mouths of the smaller rivers. A fine example is Pointe Isère, a mud cape that deflected the mouth of the Mana River in French Guiana. During inter-bank phases, higher wave energy can result in rapid and massive shoreline erosion, except where sandy-shelly cheniers develop from winnowing of coarse-grained bedload from the surrounding muddy matrix. In order to contribute to a better understanding of the rates and scales of shoreline change on the French Guiana coast, we conducted a GIS analysis on Pointe Isère using a >60-year record of aerial photographs and satellite images. The results show significant and quasi-continuous erosion of this mud cape at the multi-decadal timescale. A phase of massive retreat of the shoreline (>1 km between 1955 and 1972), was followed by much slower erosion up to 2015, probably due to: (1) the alongshore passage of several mud banks, and (2) the formation of a sandy chenier through concentration, by waves, of contemporary sand of fluvial origin stored in the inner mud-dominated shoreface, and of sand derived from erosion of an old inland chenier exposed as a result of shoreline retreat. The new chenier has also been characterized by a westward-elongating spit. The progressive demise of Pointe Isère finally resulted, in 2001, in a breach that created a new direct outlet for the Mana River, and sealing of the former outlet through downdrift distal spit welding onshore. Over the study period (1955-2017), the demise of Pointe Isère has involved a loss of 41.8 km2, much of it mangrove wetlands. This corresponds to about 8% of the total area of mangroves in French Guiana in 2015. Since 2015, a new phase of large-scale muddy accretion has been associated with the ongoing isolation of the active sandy chenier. This probably heralds a new cycle of mud-cape formation in the vicinity of the mouth of the Mana that will lead to a new westward diversion of this river. During the continuous erosion of Pointe Isère, the longshore passage of several mud banks did not mitigate or counter erosion. This has important implications regarding the unpredictability of shoreline change on the Amazon-Orinoco coast, and the need to go beyond the common vision of such change in terms of mud-bank (shoreline accretion) and inter-bank (shoreline erosion) phases that cover timescales of the order of years to a decade