Rivers Drying Up in the Olomouc Region: Fish Are Dying, Ecosystems Need Up to Two Months to Recover

In the Olomouc Region, scientists have recorded critically low water levels and drying riverbeds amid drought and prolonged heat. According to hydrobiologist Martin Rulík, around four weeks without water can lead to the complete die-off of all aquatic organisms in a riverbed.

Even once water returns to a river, the ecosystem doesn't bounce back right away. "After a stream is reflooded, life can take a month and a half to two months to recover. The longer a riverbed stays dry, the longer the recovery takes," Rulík warned.

Water has warmed to nearly 30 degrees

A separate concern is the sharp rise in water temperature caused by the combination of drought and heat. In the surviving pools of the Trusovický stream near Olomouc, experts measured temperatures of 17.1–17.3 degrees, while in the Bystřice River it reached 22–23 degrees. At the natural bathing spot in Poděbrady near Olomouc, the water warmed to nearly 29 degrees Celsius, compared to a usual summer maximum of 24 degrees. In the Bystřice, water temperatures typically stay within 19–20 degrees in summer.

Fish face mass die-offs

Fish are among the hardest hit by the low water levels — as the water recedes, species diversity shrinks along with it. Fish typically move downstream and gather in the remaining pools. The combination of high temperatures, large numbers of fish crowded into a small area, and a lack of oxygen can trigger a mass fish kill.

According to Rulík, a dissolved oxygen concentration of around three to four milligrams per liter is generally considered critical for fish, though it varies by species. Salmonids are the most demanding — brook trout and rainbow trout, for instance, need around six milligrams of oxygen per liter. For aquatic invertebrates, the survival threshold is roughly two milligrams per liter.

The heat isn't the only cause

Experts say the low water levels in the Olomouc Region's rivers stem not only from the dry weather but also from unfavorable trends in groundwater reserves. This was driven by low spring flows in the Morava River, linked to the small amount of snow in the mountains over winter. Specialists say only days of sustained rainfall can meaningfully improve the situation.

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Around Prague, based on reporting by ekolist.cz