Dependent upon climatic and hydrologic conditions, the duration of filling of pit lakes may range from years to many decades following mine closure. There are three main reasons to prefer fast filling for pit lakes: (i) reduction of slope erosion and landslide frequency, (ii) abatement of acidification, and (iii) shorter waiting time for beneficial end-use of the post-mining landscape including the pit lakes. These reasons have driven the use of river and active mine dewatering water for filling of pit lakes in former lignite strip mines in Germany over the last 35 years. River water has been the most important water source for filling pit lakes created after 1990. From 1990 to 2010, ca. 2.317 let m³ of river water and ca. 763 106 m³ of water from dewatering operations of active mines was diverted into mine voids to create pit lakes in eastern Germany. Fast filling allowed for steeper slopes of the mine voids before lake filling, saving money by shaping slopes of shallower inclination. Acidification was prevented or neutralised as well. Diversion of river water was also successfully applied as a strategy to prevent acidification of already filled pit lakes although, in some cases, additional chemical neutralisation was still necessary. The most relevant processes are introduced and the German experiences in using river water and mine water for filling and management of pit lakes are presented. Examples from other countries are also presented briefly and summarising conclusions are drawn.