The Spree River, which flows through parts of Brandenburg and Berlin, has developed a striking reddish-orange color over many kilometers. This unusual coloration is caused by a natural process called Verockerung, in which pyrite — a type of iron sulfide — in disturbed soils from old lignite (brown coal) mines oxidizes. This oxidation releases iron and sulfate into the river, changing its color and affecting its water quality. The Spree is a vital source of drinking water for Berlin, and during dry summer months, up to 75 percent of its flow in the city of Cottbus comes from water pumped from closed lignite mines in the Lusatia region. Lusatia, a region in eastern Germany, has been heavily mined for lignite for decades. Since 1990, 23 artificial lakes have been created in former mine areas, covering a total of 14,000 hectares. These lakes are now being developed for recreational use, with plans to connect ten of them through canals to create 7,000 hectares of continuously navigable water. Four canals are already completed, and six more are under construction. The public company LMBV has been managing the reclamation of these former mining areas since the 1990s and is working on developing around fifty large post-mining lakes, of which 24 are located in Lusatia. Lignite mining required the pumping of groundwater from aquifers to keep mine areas dry. This practice continues today, and as mines fill with water, the aquifers rise through the disturbed soil, exposing layers of iron and pyrite to air. This exposure leads to oxidation, releasing iron and sulfate into rivers and other water bodies. To mitigate the impact of this process, retention devices have been installed in low-flow areas of the Spree. These structures help trap iron oxyhydroxides, which in turn adsorb soluble reactive phosphorus, reducing its concentration in the river. Berlin relies heavily on a water treatment method called bank filtration, where river water is naturally filtered through the soil before being pumped for drinking water. This process has supplied about 60 percent of the city’s drinking water for over seventy years. However, the increasing levels of sulfate from mine water are beginning to affect some water treatment plants. To counteract this, these plants now sometimes mix in groundwater that has lower sulfate levels during the drinking water extraction process. Although the planned closure of all lignite mines in the region by 2038 should significantly reduce the volume of mine water entering the Spree, the oxidation of pyrite in the soil will continue to release iron and sulfate for many years after mining stops. The exact timeline for this process remains uncertain. A study by the German Federal Environment Agency estimates that nearly 58 billion cubic meters of water have already been discharged into the Spree since the 19th century. The Spreewald, a network of canals designated as a UNESCO World Heritage Site, is showing signs of drying, highlighting the ongoing struggle to maintain a balance between the flooded former mines and the natural flow of the river.