The value of lichenometry and historical archives in assessing the incision of submediterranean rivers from the Little Ice Age in the Ardeche and upper Loire (France)

The geomorphologic impact of the Little Ice Age (LIA) was determined on two French Mediterranean rivers, the upper Areche and the upper Loire. In order to evaluate the impact of the LIA on the hydrology of these rivers, two historical flood chronicles were made from historical sources. The LIA can be divided in three phases of high activity (1530-1700; 1750-1810; 1840-1910). A geomorphologic study of the two rivers shows that incision is the principle process at work in the 20th century. In order to date the low terraces and bedforms present in both valleys, lichenometry was used. Rhizocarpon geographicum allowed the main features on the bottom of the valley to be dated using two growth curves made for the two studied areas. In the Ardeche, lichenometric dating showed a progressive evacuation of the inherited alluvial stock. The oldest lichens found on the foodplain and the outcrops indicate that incision began between the second half of the 17th century and the end of the 18th century, in the mid Little Ice Age. Incision stopped on the bedrock in the second half of the 19th century. In the Loire, the narrowness of the gorge did not allow sediment to be stored during the first half of the LIA. Very old lichens (> 1000 yr of age) are present on the bedrock close to the bed. However at a number of locations, the valley slightly widens, allowing sediment that was transiting through the gorge to be stored. The geomorphologic features that characterise these basins clearly show a hydrosedimentary behaviour different before and after the beginning of the 20th century. (C) 2007 Elsevier B.V. All rights reserved.