In the 1960s and 1970s, the northern part of the Rur was heavily polluted by the tailings of many German coal mines. Neither fish nor other organisms could be found, and it was dangerous to swim in the river. Foam flakes regularly flooded parts of the city of Roermond. After the closure of the mines, the waste water treatment in Germany and the Netherlands greatly improved. Only the lower part of the river is still contaminated. The water in the upper part of the river is so clean that trout and more than 30 species of fish are back. After an absence of 125 years, salmon returned to the Rur in 2004.