50% of the river does not flow all year round


Most rivers do not consist of springs, estuaries and a continuous flow of water between them. More than half stop sinking at least one day a year. This is the result of the detailed work of the EcoFlowS laboratory from” National Research Institute of Agriculture, Food and Environment (Inrae) i McGill University in Montreal (Canada) just published Nature. “We need to review our definition of what a river is: consider that we have a special ecological environment with animal and plant species adapted to the temporary absence of water. As these rivers are an integral part of hydrographic networks, we also need to review our way of managing watercourses.” believes Thibault Datry, who coordinated this work.

The planet has 64 million kilometers of waterways. The people most people visit run all year round, either because their supply is sufficient, or because people have arranged them that way, especially by building dams. “But we know very little about all the hairy rivers upstream of the basins and it’s not always obvious that someone is present to see that there is no more flow or water. Part of the year,” explains Thibault Datry. How then do you know what is going on through these millions of miles? There is a database, “Global Leakage Data Center”, which has a long history of collecting data from 5,600 flow measurement stations worldwide. Doctoral student Mathis Messager inspected them to determine a “zero” indicating the fact that not a single liter of water had passed that day, while checking that it was not a device malfunction.

A phenomenon that affects all climate zones, even the most rainy ones

The result of this work by Romain was crossed with a hydrological atlas developed by McGill University. This document compiles various parameters of the world’s rivers: catchment area, climate, reliefs, precipitation rate, human activities, etc.

The model constructed in this way enables the reconstitution of the flow throughout the year from two criteria that explain the intermittency of the river: temperature and precipitation levels. “We checked the validity of our algorithms by comparing them with WAVE, the French River Monitoring Network, where agents from the French Office for Biological Diversity regularly visit more than 3,200 observation posts “, continues Thibault Datry.

Broken rivers in Europe. In France, 35% of rivers do not flow throughout the year, mainly in the karst networks of the southeast, in the Adour-Garonne basin and in the Charente. Copyright Mathis Messager

This process today allows us to say that rivers mostly do not flow all year round. “It’s the rule, not the exception,” claims the researcher. This phenomenon affects all climate zones, including the wettest ones like the Amazon. In addition to temperature and rain, river drainage also depends on geology. This is especially the case for courses that cross layers of limestone. The model takes into account only these natural parameters. Water pumping in progress and in alluvial aquifers by people for their agriculture and drinking water needs do not count. The occasional ones are probably even more developed because we can now see that rivers as important as the Colorado in the United States and the Yellow River in China no longer reach the sea in summer, when retreat is very important.

Public policies only take into account rivers that flow throughout the year

This observation should make it necessary to review a large number of public policies. Thus, French management methods, such as European rules established Water Framework Directive (WFD), which envisages the establishment of “good ecological status” of water bodies by 2027, takes into account only year-round courses. “They don’t take into account all the aquatic fauna and flora that have adapted to the discontinuity, either by going to bed to wait for the water to return, or by moving away from the shelter for a quick recolonization of the riverbed. As soon as the waves return,” explains Thibault Datry. These environmental changes are not without climate consequences. For example, organic matter accumulated in dry riverbeds emits significant flows of CO2 during water replenishment, flows that can represent 10% of the emissions produced by perennial rivers in a single recharge episode. The next scientific step will be to implement a faunal and floristic inventory of these environments, while modeling the development of intermittency under the influence of climate change. In fact, it is likely that an increase in temperature will prolong the drying time of the layers.

Another fear of consequences in France: degradation. Not flowing all year round, these rivers could not be considered an integral part of hydrographic networks, which would deprive them of any special protection under French water law and WFD. The very definition of watercourses is still being debated and is not yet regulated, although many management rules depend on it, such as combating the spread of pesticides in the environment. It will take time to get used to the idea that a river is not just water.



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