Pielinen is the fourth largest lake of Finland, with a drainage basin area of 12,823 km2 (4,951 sq mi) equally distributed between eastern Finland and Russia. The creation of the lake and its outlet is attributed to a post-glacial isostatic rebound, which resulted in uplift of the land. As is common in Finnish lakes, the lake's color is dark, due to the high proportion of bogs present in the catchment of the drainage basin that drains humic substances.

Post-glacial rebound is the rise of land masses after the removal of the huge weight of ice sheets during the last glacial period, which had caused isostatic depression. Post-glacial rebound and isostatic depression are phases of glacial isostasy, the deformation of the Earth's crust in response to changes in ice mass distribution. The direct raising effects of post-glacial rebound are readily apparent in parts of Northern Eurasia, Northern America, Patagonia, and Antarctica. However, through the processes of ocean siphoning and continental levering, the effects of post-glacial rebound on sea level are felt globally far from the locations of current and former ice sheets.