Alluvial rivers can show unpredictable channel changes and humans living along the river corridor repeatedly have to cope with the alterations of their physical environment. This was specifically the case in the mid-sixteenth century, when the Viennese were confronted with one major problem: the Danube River successively abandoned its main arm that ran close to the city and shifted further north. This was at the time when Vienna became the permanent residence of the Holy Roman Empire (except from 1583 to 1611 when it was moved to Prague), due to the Habsburgs holding the crown. Vienna was also the capital of the so-called Danube Monarchy, which came into being in the early sixteenth century. The city assumed increasing significance, being home to and hosting important authorities and persons. In particular after the first siege by the Ottoman army in 1529, the resource need for a complex fortification system was extremely high. This paper aims to highlight: (1) the morphological Danube dynamics together with floods and extreme weather situations in the sixteenth century; (2) the main actors in the trans-formation of the Danube; (3) changes of the river's course from 1550 to 1600/1650 and the consequences for bridging the river and the infrastructure as a whole; (4) the massive engineering measures that were undertaken in order to secure Vienna's requirements in the sixteenth century; (5) the question of floodplain settlements, and (6) the contested use of resources on the Viennese Danube floodplain.