The Postomia River, a left-bank tributary of the Warta River, is 34.5 km long. It flows out of the sources in the southern slope of Glinik near Trzemeszno Lubuskie. The small stream flows in the direction of Sulęcin, through the area of the military training ground in Wędrzyn, Ostrów, Sulęcin, and into the Postomski Canal in Krzeszyce.
Since 2005 the territory of the riverside bank with its flora and the naturally shaped river basin has been a nature reserve called the Postomia River Valley. 183 species of rare plants grow in the area of 96 ha.
The Postomia River basin covers the area of approximately 200 km2. It is the largest river in the Sulęcin Region and its impact on the surrounding environment can be seen even in the nomenclature. The Postomski Forests around Sulęcin owe their name to the river.
Fish species typical for so-called mountain rivers, such as the brown trout and pike, live in the river. In its part between Trzebów and Krzeszyce one can also find such species as the carp and roach.
An 11 km fragment of the Postomia River has been regulated. A promenade has been built along the fragment of the river flowing through the centre of Sulęcin; it is a perfect recreation place. The Polish-German Cooperation Centre located in the former House of the Order of St. John is also situated near the river.