Stream incision and sediment wave consecutive to three centuries of timber floating in the Morvan Massif (Central France)

The upper catchment of the Yonne River is nowadays usually considered as having been scarcely impacted by human activity. At the Seine basin scale, the gravel bed streams flowing down the small forested massif of the Morvan are even considered as of high ecological quality. However, this perception of the river changes completely once one looks back to the recent past. Indeed, these streams have been subject to intensive industrial activity for more than 300 years, between the 16th and early 20th centuries, as Paris’ need for timber fuel amplified. The Yonne River and all of its tributaries were heavily modified to facilitate the transportation of timber logs towards Paris through floating on the Morvan’s dense network of streams and the Yonne’s and Seine’s main channel. This activity has led to intense modifications of sediment flux still easily observable on the present river morphology, 90 years after the floating activity ended.
Every single stream of the upper Yonne catchment was equipped with small ponds allowing the generation of water releases, flushing the logs downstream. Historical archives allowed the discharges and the frequency of these flushes to be calculated. Artificial floods developing specific stream powers of over 250 W/m² were generated several times per week during wintertime in steep-sided streams that were 4 to 5 m wide. Such energy generated a drastic increase in sediment transport and led to erosion and massive incision of the beds. A few kilometers downstream, when the small tributaries joined the main valley, the Yonne River had a larger bed and gentler slope. Artificial floods were thus less powerful there (specific stream powers lower than 80 W/m²) and sediment transport conformed more to natural conditions. Considering the huge amount of sediment supply from upstream and the slowing down of the sediment flux, the Yonne river bed aggrades. A perched riverbed today lies up to 1.5m higher than the floodplain and may be seen on more than 25 km of the course.