Isère (river)

The Isère (, ; Isera; Isèra) is a river in the Auvergne-Rhône-Alpes region of southeastern France. Its source, a glacier known as the Sources de l'Isère, lies in the Vanoise National Park in the Graian Alps of Savoie, near the ski resort in Val-d'Isère on the border with Italy. An important left-bank tributary of the Rhône, the Isère merges with it a few kilometers north of Valence.

Many riverside communes have incorporated the Isère's name into their own, for example, Sainte-Hélène-sur-Isère and Romans-sur-Isère. The department of Isère is likewise named after the river.

Etymology
The name Isère was first recorded under the form Isara, which means "the impetuous one, the swift one." Not originally a Celtic word, it was very likely assimilated by the Celts in ancient times. This word is related to the Indo-European *isərós, meaning "impetuous, quick, vigorous," which is similar to the Sanskrit isiráḥ इसिरः อิสิระ with the same definition. It was probably based on the reconstructed Indo-European root *eis(ə) (and not *is), which incidentally has not been found in the Celtic languages of the British Isles.

The word Isara figures in the etymology of many other river names, from ancient Gaul and its neighboring lands. Examples of this are the Ésera in Spain, the Isar in Germany, the small Franco-Belgian Yser, or even the ancient name of the Oise, Isara (the French adjective isarien still exists in the language and continues to describe anything related to the Oise). In non-Celtic countries, we find the Isarco, a river in Northern Italy, the Éisra and Istrà in Lithuania, Jizera in the Czech Republic and Usora in Bosnia and Herzegovina.

Geography
The Isère's course measures 286 km and runs through a wide variety of landscapes: from its source near the Italian border in the western Alps, it crosses the Pays de Savoie and the Tarentaise Valley, cuts between the Chartreuse and Belledonne mountain ranges, follows the Vercors Massif, passes through the Dauphiné province, and finally meets with the Rhône at the foot of the Vivarais.

Valleys
The upper valley of the Isère is called the Tarentaise, and its middle valley the Grésivaudan.

The lower valley constitutes a section of the (also called the Valentinois) and is characterized by the river's deep, winding channel. Instead of widening its banks over time, the Isère has dug deeper into its bed, forming stepped fluvial terraces. The valley has clearly defined borders and is relatively narrow, not exceeding 2 km in breadth.

The repetition of alluvial deposition (during periods of Quaternary glaciation) and overdeepening (during interglacial periods), known as a fluvioglacial system, led to the formation of several stepped terraces in the lower Isère valley, like the one on which Saint-Marcel-lès-Valence is built. This occurred through the massive accumulation of alluvium from the Isère on top of a bed of Miocene molasse. Today, these terraces still define the geography of the Plain of Valence.

Confluence
The Isère initially merges with one of the Rhône's diversion canals, built for navigational purposes, at Pont-de-l'Isère. At the southern tip of La Roche-de-Glun (a commune on an island formed by the canal), the Isère Dam drains part of the water back into the Rhône and permits the Isère to continue its course alone until it passes through the Bourg-lès-Valence Dam and reaches its final junction with the Rhône.

Main Tributaries
(L) Left-bank tributary; (R) Right-bank tributary.
 * (L), 38.7 km
 * (R) Doron de Champagny, 15.9 km
 * (L), 20.9 km
 * (L), 28.6 km
 * (R) Arly, 34.5 km
 * (L) Chaise, 24 km
 * (L), 24.1 km
 * (L) Arc, 127.5 km
 * (L) Drac, 130.3 km
 * (R), 32.9 km
 * (L), 25.6 km
 * (R), 40.1 km
 * (L), 32.1 km
 * (R) Romanche, 78.4 km
 * (L), 34.6 km
 * (R), 17.2 km
 * (R) Morge, 27.2 km
 * , 25.3 km, via the Morge Canal
 * (L), 43.1 km
 * (L), 32 km
 * (R), 40 km

Cities on the Isère

 * Savoie: Val-d'Isère, Bourg-Saint-Maurice, Aime, Moûtiers, Albertville, Montmélian
 * Isère: Pontcharra, Grenoble, Voreppe
 * Drôme: Romans-sur-Isère, Pont-de-l'Isère, La Roche-de-Glun

Hydrology
The length of the Isère is 286 km, and its drainage basin covers 11890 km2.

The vertical profile of the river is made up of several zones:
 * From its sources to Sainte-Foy-Tarentaise (except for those sources having a slope of around 25%), the average slope of the Isère is 5.1%, in a more or less confined valley (forests, gorges, and higher up, grasslands).
 * As far as Moûtiers, the slope measures 1.18%.
 * Before the river's confluence with the Arly, its slope is only 0.53%.
 * The slope decreases to 0.136% until Grenoble.
 * Downstream from Grenoble, it measures 0.1%.

The flow of the Isère was observed over a period of 58 years (between 1956 and 2015) at Beaumont-Monteux in the Drôme department, situated near the river's confluence with the Rhône. The discharge of the river at Beaumont-Monteux measured 329 m3/s.

The Isère's large seasonal fluctuations are typical of rivers fed in large part by snowmelt, with springtime flooding raising the average monthly discharge between 382 m3/s and 500 m3/s from April to July (peaking in May and June), and low water levels in autumn and winter, from August to February, with a minimum average monthly discharge of 246 m3/s in September. Generally speaking, this makes the Isère a very plentiful watercourse throughout the year.

However, the can drop to 110 m3/s during a five-year dry spell, which is very low.

On the other hand, severe flooding can result from rapid thaw or torrential autumn rain. In fact, and QIX 5 are 1200 m3/s and 1500 m3/s, respectively. QIX 10 is 1700 m3/s. QIX 20 reaches 1900 m3/s, while QIX 50 rises to 2200 m3/s, which is still moderate compared to other rivers in the south of France, like the Tarn.

The highest instantaneous discharge on record was 2050 m3/s on September 16, 1960, while the highest daily value was 1510 m3/s on October 7 of the same year.

The depth of runoff for the Isère's drainage basin is 882 mm annually, which is quite high above France's average and clearly superior to that of the Rhône's drainage basin (666 mm in Valence for a surface area of 66450 km2 ). The specific discharge is 27.9 liters per second per square kilometer of drainage basin.