The aquatic habitat is very prevalent in the reserve as it is bordered by the Old Rhine and the Rhine canal, crossed by a former arm of the Rhine, the Bauerngrundwasser, and dotted with ponds.

The Old Rhine

The former natural course of the Rhine, now called the Old Rhine, borders the eastern part of the island and forms the border with Germany. Its flow is regulated by the Strasbourg dam upstream and the agricultural dam downstream. Situated on the Rhine-Rhone migration route, the Old Rhine forms a large body of water, which is particularly welcoming for overwintering birds.

The Bauerngrundwasser

The entire length of the nature reserve is crossed by the Bauerngrundwasser. This former meandering arm of the Rhine is representative of the Giessen in the section of the river subject to anastomosis, which accommodates the Rhine's floodwater (in summer) and is supplied by clean water from the water table during periods of low water. But the Bauerngrundwasser is no longer really a Giessen as regular flooding on the Rhine no longer exists. Today, the Bauerngrundwasser has lost all vitality and its flow is not sufficient to evacuate the silt that stifles the river bed, leaving this water course particularly eutrophic.

The river is only subject to flooding on rare occasions, inundating the Rohrschollen by means of the flood routing procedure. This precautionary flooding is used to kick start the site's hydrosystem by replenishing the water table and depositing fertile silt. The current flooding problem on Rohrschollen Island is the phenomenon of the bathtub effect, which has the tendency to asphyxiate the habitat. Despite infrequent flooding, the natural habitat is tending to dry out, bringing about a change to the alluvial habitats.

Humid areas

Ten or so ponds and temporarily flooded areas can be found in the depressions (or Gründe) on the reserve. These are replenished by rainwater or the water table. They are home to a wealth of diverse fauna, such as dragonflies and amphibians.