Alpine rivers are working for us

Study site

The Upper Drac is a gravel-bed braided river of the Southern French Alps, draining the southern part of the Ecrins National Park. It is one of the major alpine tributary to the Isère River. The study reach is located close to St-Bonnet-en-Champsaur at an elevation of 1000 m above sea level. This reach drains a 340-km² upland catchment with a maximum elevation of 3441 m. The catchment geology is composed of a complex assemblage of crystalline and sedimentary rocks.

The study reach extends from the Champsaur leisure center to the village of St-Bonnet-en-Champsaur. This 3.7 km reach presents a mean active channel width of 110 m and a mean channel slope of 0.01 m/m. The channel morphology is an artificial wide and shallow channel with a rectangular cross-section recreated by the restoration project; it is expected to rapidly transform into a braided channel. The water discharge is monitored since 1972 at the Ricous gauging station, located 14 km upstream from the study reach. The water regime is characterized by a major snowmelt peak during May and June. The mean daily discharge is 5.46 m3/s, and the 2 and 10 yr daily flood discharges are estimated at 41 and 61 m3/s, respectively.

General relief map of the Drac catchment, main physical features at the restoration site, a view of the restored reach of the upper Drac river near St-Bonnet-en-Champsaur and the geoportail view of the restoration site (©Irstea).

Restoration project

Like most of alpine braided rivers in France, the Drac has been highly impacted by intensive gravel mining since the late 1960s (Liébault et al., 2013). The gravel mining activity in the Chabottes plain was active from 1970 to 2012, at a rate exceeding 15 000 m3/yr (Laval and Guilmin, 2014). Another mining site was active at St-Bonnet during the 1970s and 1980s, generating regressive erosion along the Drac. The Upper Drac is not impacted by any dam, and it is marginally affected by embankments.

These human alterations of the sediment regime resulted in important channel responses, like active channel narrowing, attested by historical aerial photographs, and channel degradation, as attested by the historical long profile of 1913 (Liébault et al., 2013). A shift from a braided to a wandering pattern can be clearly observed along several reaches, including the study reach. Near St-Bonnet, the incision reaches 2 to 4 m, and propagates upstream (Laval and Guilmin, 2014). This incision rapidly cut through the relatively thin alluvial layer, and starts to scour lacustrine clay deposits from the Last Glacial Maximum. Once this clay layer has been reached, the incision dramatically accelerates, and a 4 to 5 m deep canyon-like channel formed along the Upper Drac. This dramatic accelerated channel incision has several consequences: (i) destabilization of the banks, with a direct threat for the artificial pond of the Champsaur leisure center; (ii) lowering of the water table and subsequent alteration of the riparian forest; (iii) alteration of aquatic habitats related to the loss of gravel substrate and to the expanding clay outcrops.