Two natural rivers, the Rhine and the Plessur, flow through the urban area of Chur. For around 900 years, however, the two man-made mill streams have also flowed through Chur, supplying the town with water and energy. These are the Obertorer and the Untertorer Mühlbach.
They are caught in the sand and supply small and large mills and businesses with mechanical and electrical energy over a length of 8.5 km. In the past, the mill streams fed the open-flowing town brooks, took care of sewage treatment, provided water for extinguishing fires and irrigating the fields, and filled the town moat when needed. From the middle of the 16th century, a small number of industrial enterprises such as sawmills, flour mills or blacksmiths were established in Chur. They also used water power and therefore settled along the two mill streams.
Tanners, furriers and fur makers practiced their trade outside the city wall at the Gerbertor, as the butcher's gate at the Arcas was also called. From the mill stream they drew clean water, while the polluted water together with the waste disappeared into the nearby Plessur.