In Lyon last Friday, in Strasbourg this Monday. The prefectures are taking stock of their rivers at the beginning of March. A Rhône which is not “inexhaustible”, according to the water agency which has just published a new study on the flows of the “most powerful river in France”. It currently offers a relatively abundant water resource. However, with climate change, the evolution of river flows by 2050 is becoming a collective concern. Collective to the point that the editorial staff of 20 minutesin Paris, Nantes, Toulouse, Strasbourg joined that of Lyon to take stock of the reserves of our five major rivers.
Inventory of the Seine, the Loire, the Garonne, the Rhine… and the Rhône for which climate projections estimate a drop of around 20% in average summer flows in Beaucaire, in the Camargue.
For the Garonne, a “worrying” situation
Last Thursday, at Lamagistère near Agen, the flow of the downstream Garonne was some 250 m³/second. A value that would be classic at the beginning of September, at the end of a torrid summer. But for the beginning of March, “we are quite simply in the lowest flow rates ever recorded over the last thirty years”, underlines Bernard Leroy, head of low water management at the Joint Syndicate for Studies and Development. of the Garonne (Smeag). With “two to three times less water” than usual depending on the meanders of the river, the specialist considers the situation already “worrying”. Even if it can still snow in the Pyrenees, or rain a lot in the plain, “this rain will not be able to completely penetrate the ground”. This winter aridity comes when the water stocks in the dams, already in high demand in the summer of 2022, are also abnormally low. In Haute-Garonne, for example, the dams were only 30% full three weeks ago.
On the Rhône side, “it’s time to make the right decisions”
“The Rhône river, even if it is the most powerful in France, is not inexhaustible. It is already affected by climate change”, released Martial Saddier, president of the Rhône-Mediterranean basin committee, last Friday by presenting a study on the flows of the river, commissioned two years ago. Over the period 1960-2020, the air temperature has increased by 1.8°C, the amount of snow has decreased by 10% and the soils are drying out further. “As a result, the low water flows of the Rhône have decreased by 7%, at the exit of Lake Geneva, to 13% in Beaucaire over the past sixty years”, detailed Laurent Roy, director general of the Rhône water agency. Mediterranean Corsica. By 2055, according to estimates, the flows of the Rhône could still drop by around 20% in the middle of summer. And the predictions for 2100 are more worrying if the glaciers in the Alps were to disappear. More than 60% of the flows of the Rhône come from the melting of this ice.
“Because we are in conditions where we are still lucky, it is time to make the right decisions”, alerted the water agency. Saving water, replenishing the soil by dewatering it, replacing certain tributaries and storing water in times of overabundance, etc. The institution recommends about sixty measures to be put in place now. “All the signals are already red concerning hydrology, recalls Marie-Hélène Gravier, head of the water service at Dreal. This year 2023 already promises to be worrying with the lowest level of snow cover for fifteen years and a rainfall deficit of 75% in certain places in the Rhône-Mediterranean basin”.
For the Seine, “we hadn’t seen that since 1989”
Thirty-one days without precipitation since January 21… The Seine basin is thirsty, and is experiencing a significant rainfall deficit, according to the Ile-de-France prefecture. As a result, the situation of rivers and lakes is “unusual for the month of February”, according to the authorities: “At the moment, the Seine in Paris is flowing at 150 m3 per second against averages in February-March at 540 m3/second. » And the soil humidity index is around 30% below normal for the season throughout the Seine-Normandy basin. “We have a very atypical winter drought, we had not seen that since 1989”, comments Vazken Andréassian, hydrologist and director of a research unit at the National Institute for Agronomic Research (Inrae). The coordinating prefect of the Seine-Normandy basin must soon bring together the 28 prefects of the basin to take appropriate measures.
This year 2023 already promises to be worrying with the lowest level of snow cover for fifteen years and a rainfall deficit of 75% in certain places in the Rhône-Mediterranean basin.
Is the Seine suffering from the effects of climate change? The ClimAware study bringing together several research institutes had shown in 2014 a very strong impact of the flow of the Seine by 2050, but recent intermediate results tend to say that the average would not change, because the rivers would be wetter in winter and drier in summer. “This remains to be confirmed by the end of the year”, explains Vazken Andréassian. On the other hand, we are well “towards longer droughts in the summer”, explains the expert. To deal with this situation, the Seine-Normandy water agency has already devoted 18 million euros to projects “accelerating the resilience of territories in the face of climate change”.
The Loire fears a “critical situation” this summer
The longest river in France had already suffered last summer from a spectacular lack of water, especially between Angers and Nantes. Less than six months later, concern is again in order for the Loire. On March 1, its flow was 390 m3/second at the reference station of Montjean-sur-Loire, four times less than an average flow at the same time. The equivalent, roughly speaking, of the beginning of summer. “It is an exceptional situation on this date, confirms Stéphane Marlette, head of the hydrology division at Dreal Pays-de-la-Loire. You have to go back a long way to find comparable readings, perhaps to the 1940s. The years 2019 and 2022 were also particularly low, but today’s flow is even lower. »
The main tributaries of the Loire, in particular the Cher, the Sarthe, the Mayenne and the Erdre, also show an abnormally low level. “If the trend does not improve, we are heading towards a critical situation this summer”, estimates Simon Lery, director of the GIP Loire Estuary, responsible for monitoring the river. The consequences would be significant for wildlife, but also for humans, as drinking water collection stations could be forced to stop. However, we are not there yet. “The flow can start to rise again and return to a completely normal situation with rainy episodes provided in the spring, recalls Simon Lery. It has been seen before, especially in 2017.
On the Rhine side, “we must remain vigilant”
A very rainy autumn and a too dry start to the year, but don’t panic. “The region’s water tables are barely below normal, as is the soil moisture index, although February suffered the third largest rainfall deficit since 1959,” said Thursday. Frédéric Charles, territorial delegate of the Bas-Rhin of the regional health agency (ARS) of the Grand-Est.
Still, the Rhine is doing “almost well”, assured the authorities together after a meeting of the Bas-Rhin water resources committee. The committee had wanted to make a “diagnosis of the situation at the local level” a few days after the Minister for Ecological Transition Christophe Béchu asked all the prefects to anticipate possible crisis situations during the summer.
Numbers ? A flow at the end of February of around 500 m3/s was recorded in Alsace, for an annual average of 1,000 m3/s. It is “a little lower than normal in this low water season, detailed Hervé Vanlaer, director of Dreal. But this is not at all exceptional, we have already seen it below. We have to remain vigilant”. Because on the one hand, the low level of snow stock in the Alps can be worrying, noted the monitoring and management organizations of the river. And because, on the other hand, “we are moving more and more from a nival regime to a river regime: that is to say with high waters that occur earlier in the year”. “As there is less snow, less snowmelt in the summer on the alpine glaciers, the level of the Rhine tends to be lower in the summer”, continues Hervé Vanlaer. As a result, he says, “for the end of spring and especially during the summer period, we risk having low water problems [débit minimal] on the Rhine with transport difficulties”.