France owes to its climate and its relief, if it is bathed by a large number of rivers, and if it possesses almost in every part the elements necessary for agriculture. The concentration in a small number of large river basins is favorable for runoff: and 4 / 5 of its territory is drained by the Rhine, the Seine, the Loire, the Garonne and the Rhone. If the annual dryness index (P: T+ 10), it can be seen that everywhere, except in the Mediterranean region, it is higher than 20, which is the characteristic value of countries where water can be lacking. The average runoff coefficient for the north of France can be calculated at 30%, i.e. almost a third of the rains that fall are carried away by rivers. The variety of outcrops of now permeable and now impermeable soils multiplies the aquifers and springs, which rarely dry up in the Parisian Basin.
Although it has higher temperatures, the midday of France has, at least for large rivers, often higher drainage coefficients than those of the north (Rhone 50%, Isère 72, Garonne 45). Indeed, the more rugged relief makes the drainage faster, the impermeable soils predominate in the Massif Central, the rains are mostly torrential, and finally the snow represents the main part in feeding the sources of the rivers. The underground water reserves, on the other hand, are less abundant and more prone to drying up in the south than in the north, except in the case of limestone thick enough to be able to deeply store water that escapes evaporation and then reappears in the form of powerful springs. like the “Fontaine de Vaucluse”.
According to Pharmacylib, the set of local influences produces several types of river regime. The sequanian regime is that of the rivers of the plains and hills, which, fed almost exclusively by the rains that fall almost the same throughout the year, flow with higher waters in the cold season, when evaporation is less, and another cause of deficit has disappeared, with the stopping of the vegetation and with the lower waters in the hot season (August-September): the difference in level is more or less accentuated, according to the importance of the underground reserves, importance that depends on the extent of permeable soils.
The Ligerian regime is that of mid-mountain rivers, which have their sources between 500 and 1500 m. (Loire and tributaries, Moselle, high Meuse, Doubs, etc.). Two facts depend on the height: faster drainage on strong slopes and winter snows that remain on the ground until spring, which therefore becomes the season of high waters, while there are two low periods: summer and winter. The monthly flow curve is always quite bumpy.
The alpine regime is that of the rivers that descend from the high Alps (Rhone, Isère, Durance). The preservation of precipitation in the form of snow lasts even longer, and glaciers, for their part, are inexhaustible reservoirs. In every part the flow increases with the temperatures and reaches its maximum in the summer, although there are sudden floods at the beginning or at the end of winter, when rains fall warm enough to melt the snow. Flow changes are frequent, and the more abrupt, the stronger the slopes.
The Mediterranean regime is that of the rivers that descend from the Cévennes and all those of Lower Provence. The drought of summer often reduces them to a thin stream of water, barely visible in a large bed of pebbles or sand, while the spring and autumn rains cause terrible floods.
Now let’s see how the river basins are arranged, referring to the individual items for the main ones. In the north of France, a region of hills and medium-sized Hercian mountains, the waters meet in three large river basins: that of the Rhine (with its Alsatian tributaries Ill and Lauter and the Moselle); that of the Seine with all its tributaries: Marne, Oise, Epte, Aube, Yonne, Loing; that of the Meuse, whose special situation and the almost absolute lack of tributaries depend on the catches made against it by its neighbors. Some secondary basins have retained their individuality: the Scheldt drains the western end of the Ardenne massif and the hills of Artois; the Sum, whose sources are separated by a slight threshold from the Oise, with which it must once have had a closer relationship, collects the waters of a large part of Picardy; another portion is instead drained by small rivers parallel to its course: the Béthune, which collects the waters of the town of Bray; the Authie and the Canche, which descend from the summit of the Artois.
The Armorican Massif has preserved an independent hydrographic network everywhere, except in the south-eastern part, where the Loire has attracted the waters of the Maine and the Perche, which are brought to it by the Mayenne, the Sarthe and the Loir, gathered at Angers. to form the Maine, as well as most of the waters of the Vendée (Sèvre, Nantaise).
From the hills of Normandy numerous fast flowing rivers descend to the north: the Touque, which runs through the village of Auge; the Dives; the Orne, whose lower canalized course gave rise to the port of Caen.
Almost all the waters of the Cotentin (Douve) and part of those of the hills of Normandy (Vire) flow into the bay of the Seine. Brittany has only two rivers of some importance: the Vilaine, which collects the waters in the Rennes basin and, after crossing the Armorican sandstone ridges with a gorge, after Redon it flows into a bridged estuary: it is a Sequanian type river, with black waters, and with a steep enough slope, to make the locks necessary for navigation; the Aulne, which flows into the bay of Brest, runs even more often encased and is easily swollen by the rains that fall on the peaks of the Arrée mountains or the Montagne Noire. All the other Breton rivers dry up only small basins, even though they end in estuaries that have risen up by the tide, after a lower course invaded by a recent marine transgression.
The Massif Central gives rise to the Loire, which, for the length of its course, is the largest river in France, and which also collects the waters of a part of the Parisian Basin and the Armorican Massif.
The Aquitaine Basin is almost entirely drained from the Garonne; but the bundle of the Gaves, which descends from the western Pyrenees, has preserved its independence, concentrating on the Adour artery. The great sandy plain of the Landes is crossed only by short and thin rivers, the most important of which is the Leyre, which flows into the Gulf of Arcachon. The great depressed furrow, which extends between the margin of the Massif Central on one side, the Alps and the Jura on the other, has naturally served as a collector for the waters; and the Rhone, with its large tributary Saone, dries up the most diverse regions due to the relief, the climate and the regime of the rivers that descend from it. Only a few rivers that descend from the Cévennes escaped it: the Vidourle, the Hérault and the Orb, which are true Mediterranean streams; and some rivers of the lower Provence: the Huveaune which irrigates the meadows near Marseille; the Argens, with its waters reddened by permic marls, which follows the great marginal depression of the Maures, as far as the bay of Fréjus; the Loup, which flows into Cannes through fantastic gorges; and, most notably, the Varo, torrential river, alpine and Mediterranean at the same time.
The Eastern Pyrenees also easily preserved the independence of their hydrography. The Têt and the Tech descend rapidly and wet the Roussillon plain, where the melting of the spring snow makes them very useful for irrigation. The Aude has a more complicated course through the limestone chains, where it is encased in strange gorges up to the threshold of Carcassonne, from which it turns east towards Narbonne: it is a mountain river, with a capricious regime, with spring floods and autumnal species.