Isar — Alpine river running from Tyrol to the Danube through Bavaria

According to current research, the name of the river can be traced back to the hypothetical Indo-Germanic root *es or *is with the meaning "(flowing) water", which in today's languages has narrowed to the solid aggregate state of water ("ice"). The river is first mentioned in 763 as "Isura" in the Traditionsbuch of the Hochstift Freising.

A number of other river names are derived from this root:

• Jizera or Iser (Czech Republic), Izera (Poland)
• Isère, (France)
• Isère, Oise (then Isara) (France)
• Isel (Austria)
• IJssel (Netherlands)
• Yser (French), IJzer (Dutch) (Belgium)
• Eisack (Ital.: Isarco) (South Tyrol, Italy)
• Isen (Bavaria, Germany)

The name Ister for the lower section of the Danube probably has the same origin. The "Eisach" ("watercourse") containing the same root as the name of several mountain streams in the Alpine region does not necessarily refer to "ice-cold" water.

Hans Bahlow's interpretation that the word Isar can be derived from es, as or os and can thus be interpreted as "swamp water" is highly controversial among experts, since the namesakes are flowing bodies of water.

In any case, the interpretation that the name Isar is composed of the Celtic words ys (fast, torrential) and ura (water, river) is considered outdated. According to another interpretation, ys stands for high and low at the same time and thus refers to the vertical.

The Isar was probably already used as a trade route in prehistoric times to transport goods from the area of the Alps and from Italy to the Danube with the help of rafts. A pre-existing trade route from the Inn valley over the Seefelder Sattel into the northern foothills of the Alps was extended by the Romans from 195 AD to become the Via Raetia. The market town of Mittenwald on the Isar River was thus able to develop from a Roman post to an important transhipment point for trade goods in the Werdenfelser Land region.

Bridges over the Isar have only been documented since the Middle Ages. The cities of Munich and Landshut were founded in the Middle Ages in connection with the construction of bridges over the Isar, and the aim was always to control trade routes and thus to gain power and economic influence. The further expansion of the towns created a constant demand for wood and lime, which led to an upswing in rafting (especially in the Oberland). From the 17th century onwards, goods such as tropical fruits, spices, cotton and silk were also transported from the Venetian market in Mittenwald via the Isar to Vienna and Budapest. At the peak of rafting in the 19th century, more than 8,000 rafts landed in Munich every year.

Since the Middle Ages, among other things, water mills were driven by the water power of the Isar, which needed a constant water level. Therefore, in Munich and in Lower Bavaria (Klötzlmühlbach and Längenmühlbach) water was diverted from the river into smaller mill channels. The Munich city streams, as canals, also served to supply the population with service water and fed the ditches in front of the medieval city walls. During the annual floods, there were repeated floods and accidents in the adjacent towns and communities. In Munich, for example, a predecessor of today's Ludwigsbrücke bridge collapsed in 1813, killing more than 100 onlookers standing on the bridge; in the 1899 flood, the Luitpoldbrücke and Max-Joseph-Brücke bridges also collapsed in Munich. Since 1806, in Lower Bavaria since about 1900, the banks began to be fortified and the river channelled so that it burrowed deeper into its bed and thereafter rarely overflowed its banks.

Further extensive, regulating measures have been carried out since the 1920s in order to generate electrical energy from hydropower. With the construction of the Walchensee power plant in 1924, massive intervention was made in the natural upper course of the Isar. Since then, the Krün weir has diverted almost all of the Isar's water to Lake Walchen. In 1951, the Isar also lost almost all of the water from the Rißbach, which has also been diverted to Lake Walchen since then. The upper course of the Isar lost further water with the construction of the Achensee power station in 1927 - the Achensee now primarily no longer drains via the Seeache/Walchen and Isar, but southwards via the Inn. Since the 1950s, water from the Dürrach has been fed to the Achensee power plant, which meant a further loss of water from the Isar. Therefore, the upper part of the Isar became more and more a river corpse, so that an urgent improvement of the situation was needed. Therefore, the then highly controversial construction of the Sylvenstein reservoir (completed in 1959) was finally started in order to achieve a more constant water level of the Isar. The additionally guaranteed flood protection, however, was only a secondary goal. In Landshut, the Landshut flood basin was completed in 1955, which absorbs part of the water during floods.

Further hydroelectric power plants with barrages were built until the 1980s, such as the Landau hydroelectric power plant in 1984. Only recently have attempts been made to restore at least parts of the Isar to its original wild river character through various measures. For example, the Isar Plan in Munich and the renaturation of the banks of the Isar near Landau and, from 2016, near Dingolfing.

In the lower reaches of the Isar between Moosburg and Plattling, gold was panned from the river deposits, especially in the 16th and 17th centuries. Place names such as Golding (municipality of Gottfrieding) and Goldern (municipality of Niederaichbach) still bear witness to this. However, the quantities of precious metal extracted were small and not of great economic importance. The gold was mainly used to coin river gold ducats. They are recognizable by the inscription EX AURO ISARE (= from the gold of the Isar).

Modern canoeing was founded on the Isar. Alfred Heurich paddled the Isar between Bad Tölz and Munich for the first time in 1905 with his self-built folding boat in the form of a kayak. This sport quickly became popular throughout Europe.