Discontinuities in braided patterns: The River Rhone from Geneva to the Camargue delta before river training

This study presents the upstream-downstream complexity of the Rhone, which earlier was a braided river. The fluvial patterns of the Rhone ca. 1860-70 are discussed, i.e. at the end of the Little Ice Age, which was also the beginning of the period of river dredging for improving navigation in the channel (1840-1930) and before the development of a chain of hydroelectric dams (1892-1986) on the river. Flowing across Switzerland and France to the Camargue delta on the Mediterranean, the Rhone drains the western part of the Alps in Europe. Befitting a large river flowing from the mountains, the Rhone was braided along most of its course, due to large quantities of coarse sediments formerly contributed by tributaries from the Alps, Jura Mountains, and Massif Central. The paper begins with a summary of palaeo-environmental studies on the Rhone, which show that the river experienced several events of fluvial metamorphosis during the Holocene. The reaches, which were braided during the late 19th century. alternated between meandering and braided patterns during this period due to the high sensitivity of the channel pattern to external variables such as changing water and sediment discharges. A discussion on the methods used to describe different aspects of the braided pattern of the Rhone from 1860 to 1870, using large-scale maps follows. Discrete reaches on the river are identified using slope, downstream variations of discharge, and stream power. These can be attributed to three factors: the tributaries (large supplies of bed load are brought by the Arve, the Ain, the Drome, the Ardeche and the Durance rivers), Quaternary tectonics, and the along-channel distance from tributary confluences. Finally, the study assesses the pre-modern conditions of the Rhone for restoring selected reaches on the modern river. (C) 2009 Elsevier B.V. All rights reserved.