Abstract
The authors studied changes in the morphometry of Lake Ostrowskie over the period of 123 years on the grounds of cartographic materials, remote sensing data, land surveying, and data referring to the water stages. The changes that occurred in the lake up to the 1960s were related to the regulation and melioration works conducted in the entire area of the Gniezno Lakeland. On the other hand, drainage of the open pits of brown coal resulted in the considerable decline of the water table by another 2.5 m in the years 2002–2009. As a result of the decline of water down to 96.4 m a.s.l. the present lake was divided into two water bodies (the eastern one and the western one) joint with a channel. This led to the decline in the lake area from 346.7 ha in 1887 to 242.0 ha, and its volume by 28.9%, out of which by 18.3% only in recent twenty-eight years.