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 Sanskritisiráh with the same definition. It was probably based on the reconstructed Indo-European root*eis, 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. 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.
The upper valley of the Isère is called the Tarentaise, and its middle valley the Grésivaudan. The lower valley constitutes a section of the 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 in breadth. The repetition of alluvial deposition and overdeepening, 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 Miocenemolasse. 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, 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.
From its sources to Sainte-Foy-Tarentaise, the average slope of the Isère is 5.1%, in a more or less confined valley.
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 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. 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 and from April to July, and low water levels in autumn and winter, from August to February, with a minimum average monthly discharge of in September. Generally speaking, this makes the Isère a very plentiful watercourse throughout the year. However, the can drop to 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 and, respectively. QIX 10 is. QIX 20 reaches, while QIX 50 rises to, which is still moderate compared to other rivers in the south of France, like the Tarn. The highest instantaneous discharge on record was on September 16, 1960, while the highest daily value was on October 7 of the same year. The depth of runoff for the Isère's drainage basin is annually, which is quite high above France's average and clearly superior to that of the Rhône's drainage basin. The specific discharge is 27.9 litersper second per square kilometer of drainage basin. .
