Connections between the Rhône River and its delta (part 1) : changes in the Rhône delta coastline since the mid-19th century
Journal title : Géomorphologie
Volume : 2
Pagination : 111-124
Publication date : 01/06/2006
On a worldwide scale, human stress on the decrease in river sediment discharges is well known. However, this impact is not clearly defined in the case of the Rhône River. Since the 19th century, the Rhône sediment load has decreased, but it still remains difficult to estimate the consequences on the coastline evolution. The aim of this study is to determine the influence of the terrigenous input on the evolution of the deltaic coast. In this paper, coastal gains and losses since 1823 have been computed by diachronic superposition of eight topographic maps. The results have been correlated with available water and sediment discharge datasets for the Rhône River covering this period. Results show a deceleration in the growth of the delta surface area between the 19th and the 20th centuries by a factor 10 (from +0.220 km2/yr to +0.021 km2/yr), as well as an increase in the influence of the active Rhône mouth sector in the growth of the total surface area of the delta (53% of the total growth during the 19th century, 136% during the 20th century). The evolution of the delta coastline therefore appears disconnected from the contemporary river sediment input. The most important sediment sources are secondary sediment deposits, such as bioclastic sands, or supplied by dismantling of the fossil prodeltaic lobes. Only the active river mouth area benefits from direct sediment input from the Rhône. This contrast explains calls for a more detailed study of this zone, proposed in a companion paper (Maillet et al., this volume).