Summer heat has arrived in Central Europe. For many people, this means heat stress. For our rivers, it means less water, higher temperatures, and less oxygen. In several regions of Austria, there have been reports in recent days of dried-up streams, low water levels, and fish die-offs. The situation is especially dramatic where smaller rivers carry hardly any water at all, or dry up completely in places. In regulated stretches with little vegetation cover, unnaturally high water temperatures compound the problem. What we experience as summer heat quickly becomes a matter of survival underwater. The situation is especially dramatic on the Mattig in Upper Austria, but parts of the LIFE IRIS pilot river Pielach are also affected, as images of dead huchen (Danube salmon) show.

Warm water, little oxygen

The warmer water becomes, the less oxygen it can hold. At the same time, many aquatic organisms’ oxygen demand increases. Fish must survive in a situation where they need more oxygen while less of it is available. Natural water bodies have varied depths, currents, and refuge areas. In regulated waters, these habitats are often lacking, and heat periods quickly become critical. Combined with low flow levels, the situation is further intensified. Water sits longer in shallow areas, warms up more, and loses even more oxygen. In extreme cases, entire stretches of waterways dry up completely.

Residual flow sections require special attention

Residual flow sections are particularly sensitive. Below diversion points, often only a fraction of the natural flow remains in the riverbed. In times of drought and heat, this can quickly lead to very shallow water depths, higher temperatures, and restricted connectivity. Structural measures are therefore essential in such stretches. At the same time, one thing holds true: river structure cannot replace missing water. Resilient rivers need both — sufficient water in the river and suitable habitats.

One example from LIFE IRIS is the Leitha, where targeted structures were created in the residual flow section: groynes, deadwood packs, scour pools, fords, gravel bars, and a structured low-flow channel. This creates more diverse habitats in a stretch where conventional widening would not make sense given the low water flow. Additionally, a fish rescue pool is also being trialled near Lichtenwörth. A deeper area of the riverbed, intended to offer fish at least a temporary refuge in such situations. In emergencies, fish can be rescued and relocated to sections still carrying water.

River restoration and integrated river space management must be an imperative of the hour in light of climate change!