Recent Morphological evolution of a headwater stream in agricultural context after channelization (Ligoire River, France)

In France, since the beginning of 20th century, rural landscapes have been completely modified by humans' activities. These practices had resulted in profound sedimentary and morphological alterations (channel bed incision, deposition of fine sediment, bank erosion, etc.), detrimental to the achievement of good water status. The present study takes place in this context. The linear (21 km) of small stream ("La Ligoire") was studied. It drains a watershed of 82 km², occupied nearly 90 % of arable lands. The objectives of the study were threefolds: (1) quantify incision and deposition processes since the channelization of the stream (1970), (2) quantify the deposition rates of fine sediment in the channel, and (3) explain the spatial distribution of these deposits. At first, compilation of historical documents (longitudinal profile, cross sections, plan profile) allowed the stream original topography reconstruction, just after channelization. In a second time, cross sections and sediment thickness were measured at the same locations. The results show that over the 1970 -2012 period; 60% of the stream linear was incised on average by 34 cm (- 0.8 cm/y) and the total volume of corresponding sediment amounted to 16 300 m3 (390 m3/y); 40 % of the stream linear had increased of + 31 cm on average (0.7 cm / y), corresponding to a volume of sediment of 12 400 m3 (300 m3/y). The process of incision and sedimentation are also related to the channel topographic evolution over the same period (longitudinal slope, width, etc.) Moreover, the volume of rather fine-grained sediments deposited on the bed of the stream channel, measured over the April-May 2012 period, amounted to 12 130 m3, an average thickness of 19 cm. The distribution of sediments is partly influenced by the presence of five water obstacle