Dzierżno Duże Reservoir

Dzierżno Duże is not a natural lake but a flooded sand pit: in 1964 the waters of the Kłodnica river were let into two quarry pits up to 20 m deep. The result is one of the largest reservoirs in Upper Silesia, about 5.8 km long and covering more than 6 km², lying between Gliwice, Pyskowice and the village of Rzeczyce. It appeals more to birdwatchers and fans of post-industrial landscapes than to beachgoers.

History: from sand pit to reservoir

The sand dug in the Kłodnica valley was used as backfill in Silesian coal mines, filling the voids left after coal extraction. When quarrying ended, the two pits, separated by a dyke, were flooded, and the dyke has since been washed away. The reservoir, also called Lake Rzeczyce, takes its name from Dzierżno, a district of Pyskowice. To the south and east its basin has a natural shoreline; to the west it is closed by an earth dam and to the north by an embankment separating it from the Gliwice Canal.

The reservoir is fed mainly by the heavily polluted Kłodnica through a three-step inlet cascade. Dzierżno acts as a natural settling basin, protects the valley from floods and improves navigation on the Gliwice Canal, and the water level can vary by as much as 9 m.

What to see

• The vast water surface, 5.8 km long and up to 1.5 km wide, with about 15.7 km of shoreline.
• A regionally important refuge for waterbirds, especially during migration.
• The Gliwice Canal running right beside the northern embankment, with the Dzierżno lock.
• A post-industrial landscape with industrial chimneys on the horizon.
• Sandy shores on the Rzeczyce side, good for a walk.

Interesting facts

In August 2024 a mass fish die-off struck the reservoir and the neighbouring section of the Gliwice Canal, linked to a bloom of the golden alga Prymnesium parvum. The provincial governor banned use of the water, and several dozen tonnes of dead fish had been removed by 16 August, with help from anglers, firefighters and volunteers. On 28 July 2026 the state water authority Wody Polskie signed an agreement for the reservoir's modernisation worth about PLN 556 million, of which PLN 443 million is EU funding; the works will include compacting the dam body and embankments.

Tripto tip

Dzierżno Duże is not a bathing lake: its water quality has been rated below all classes, and since 2024 Wody Polskie has taken protective measures against golden alga spreading from the Gliwice Canal. Before you go, check the notices of Wody Polskie in Gliwice and don't let your dog into the water. The best access to the shore is from Rzeczyce and Pyskowice, and a walk along the embankment overlooking the canal and the lake takes 1–2 hours. Do not climb onto any water-management structures, and expect parts of the embankments to be closed during modernisation works. Fishing follows the rules of the local Polish Angling Association district, so check the current fishery regulations before you go.