Do you know why the Rhône, so opaque at Bouveret, emerges from Lake Geneva almost translucent?
The answer to this question lies in the 𝗱𝗲𝗹𝘁𝗮 𝗱𝘂 𝗥𝗵𝗼̂𝗻𝗲. The milky colour of the Rhône is due to the presence of suspended sediments. At the mouth of the Rhône, these particles settle to the bottom of the lake, forming what is known as a delta.
Like the Nile delta (probably the best known), deltas are distinguished by a triangular shape close to the Greek letter Δ (delta), from which they take their name. 𝗜𝗹𝘀 𝗷𝗼𝘂𝗲𝗻𝘁 𝘂𝗻 𝗿𝗼̂𝗹𝗲 𝗺𝗮𝗷𝗲𝘂𝗿 𝗽𝗼𝘂𝗿 𝗹𝗮 𝗯𝗶𝗼𝗱𝗶𝘃𝗲𝗿𝘀𝗶𝘁𝗲́ by offering habitat conditions sought after, particularly by many bird species.
As well as providing a natural transition zone, the Rhône delta is very important 𝗱𝗮𝗻𝘀 𝗹𝗲 𝗽𝗿𝗼𝗰𝗲𝘀𝘀𝘂𝘀 𝗱'𝗼𝘅𝘆𝗴𝗲́𝗻𝗮𝘁𝗶𝗼𝗻 for the different layers of water in Lake Geneva.
As we mentioned in a previous 60 seconds, Lake Geneva is divided into three layers, from top to bottom: Epilimnion, metalimnion and hypolimnion. Warmer for most of the year, the surface waters are less dense than those in the depths. This difference in density explains the 𝘀𝘁𝗿𝗮𝘁𝗶𝗳𝗶𝗰𝗮𝘁𝗶𝗼𝗻 𝗱𝘂 𝗹𝗮𝗰.
When it enters Lake Geneva, the Rhône flows through the metalimnion (interflow process). In fact, the temperature of its water is similar to that of this layer. It can also happen that the Rhône water, loaded with sediment, is denser than the hypolimnion, producing 𝘂𝗻 𝗰𝗼𝘂𝗿𝗮𝗻𝘁 𝗱𝗲 𝗳𝗼𝗻𝗱 (underflow).
𝗟𝗲𝘀 𝗿𝗮𝘃𝗶𝗻𝘀 𝘀𝗼𝘂𝘀-𝗹𝗮𝗰𝘂𝘀𝘁𝗿𝗲𝘀 𝗱𝘂 𝗱𝗲𝗹𝘁𝗮 𝗥𝗵𝗼̂𝗻𝗲 have been formed by these bottom currents for over 1,000 years. These currents carry the water from the Rhône down to the bottom of the lake, providing oxygen to the water, which is struggling to be replenished by natural mixing.
Over the last 150 years, 𝗱𝗲𝘂𝘅 𝗴𝗿𝗮𝗻𝗱𝘀 𝗽𝗿𝗼𝗷𝗲𝘁𝘀 𝗱'𝗶𝗻𝗳𝗿𝗮𝘀𝘁𝗿𝘂𝗰𝘁𝘂𝗿𝗲 have greatly narrowed the Rhône delta. First of all, the first correction of the Rhône canalised the river, making it follow the course we know today by draining the other six arms of its delta. Subsequently, the growth of hydraulic dams reduced the supply of alluvium necessary for the delta's ecosystem and the oxygenation of the lake bed.
Today, the ecological role of the delta is better recognised. As a result, work to correct the Rhône River is set to leave more room for the natural functioning of the river by 2025.
Photos: JM Zellweger & Map.Geo
𝐒𝐨𝐮𝐫𝐜𝐞𝐬 :
- Le Temps(14 September 2016)
- Le Temps(7 September 2017)
- Une info en 60' n°9 Un Léman à trois étages".
- LOIZEAU (1991), "La sédimentation récente dans le delta du Rhône, Léman: processus et évolution", PhD thesis: Univ. Geneva. (online)
