Geomorphological Effects of Land Reclamation in the Eighteenth Century at the Mouth of the Leyre River, Arcachon Bay, France
Keywords:Salt marsh, coastal wetland, historic evolution, tidal lagoon delta, map analysis, river regime, ancient maps
During the second half of the 18th century, large parts of the salt marshes at the Leyre River mouth in Arcachon Bay were converted into salt pans, while brackish wetlands and mobile sand dunes, south of the lagoon, were reclaimed and/or fixed by pine tree plantations. Morphological and hydrodynamic changes in the Leyre river mouth area are due to reclamation. The method used, based on map analysis, includes the comparison of digital anamorphosis, obtained from maps surveyed during reclamation (1774-1776) and 50 years before (1708) and 20-40 years (1826) after the completion of the project including planting. Morphological changes based on map analysis are compared with published core descriptions and related to hydrodynamic changes, viz. between 1708 and 1774 the position of the channel was stable, between 1776 and 1826 the channels moved northwards (up to 800m) and the two river distributaries and their related channels at the lagoon outlet are greatly modified. Results of map analysis are compared to published core descriptions and related to hydrodynamic changes recorded in core stratification studies. Between 1774 and 1826 the fluvial influence increases in the north distributary, which previously (1708) had mainly a tidal evolution. The evolution observed after 1776 is the consequence of land reclamation. These results show that the distributary regime changed in a very short span of time, occurring immediately after land reclamation. Indirect consequences of groundwater level lowering and the improvement of surficial water circulation obtained by planting pine trees probably contributed more to this rapid evolution than the geomorphological effects of salt pan construction at the river mouth. Sedimentary sequences were previously assumed to have been deposited after 2500 BP; it now seems possible to give them reliable dates of less than 200 years.