Dynamique fluviale de la Garonne à l'anthropocène : trajectoire d'évolution du tronçon fluvial compris entre les confluences de l'Ariège et du Tarn (Garonne toulousaine, 90 km)

摘要

Since the end of the nineteenth century, significant morphological channel changes have been observed along European rivers. The major processes involved, channel incision and narrowing, appear as the result of increasing human activities and the transition from the Little Ice Age (LIA) to Global Warming (GW). On the Garonne River (southwestern France) this phenomenon has been observed and described since the 1960th between the Ariège and Tarn tributaries (The Toulouse Garonne, 90 km). However, very few studies have focused on a longer evolutionary period. In this study, we propose to analyze the evolutionary trajectory of the Toulouse Garonne on a plurisecular timescale. The aim of this study is to assess: 1) the evolutionary tendency before the 1950th; 2) the impact of the LIA to GW transition and former human disturbances on fluvial dynamics; 3) the hydro-sedimentary pattern of the Garonne River before the twentieth century; and 4) the presence of spatial discontinuities within the reach. The analysis is based on two types of data, which are complementary in terms of chronology and spatial cover: historical maps and field data collected on 11 palaeochannels. The results show: 1) a meandering channel in the late LIA (second half of the nineteenth century), with spatial discontinuities along the Toulouse Garonne; 2) a morphological adjustment since the late nineteenth / early twentieth centuries in the upstream and middle reaches, in response to several controls and; 3) a later adjustment in the downstream reach, in response to bank protection. Furthermore, this study provides new data on morphological channel features over the Medieval Climatic Optimum (MCO, 900-1300) and reveals that only a very few data allows to assess the channel behavior over the LIA.