Hydrological properties of Kozjanski Park are mostly determined by relief, geology and climate. The area belongs to the Sava river basin or to be precise, to its left tributaries the Sotla and the Savinja.
The Sotla has an extremely asymmetric river basin. The right tributaries prevail (the Olimščica, the Virštanjski potok, the Buča, the Bistrica, the Bizeljski potok, the Dramlja). The Brestanica, the Močnik, the Dramlja and the Gabrnica spring from the southern slopes of East Posavje Hills and finally join the Sava.

The Bistrica and the Gračnica valley form a central east-west axis of Kozjanski Park. Most hydrological points of interest are related to those two river sheds. The Bistrica, the Sotla, the Buča, the Dramlja, the Suhadolski potok and the Gabrnica are considered as natural values in almost in almost entire flow. Waterfalls, river rapids and thresholds enrich most streams and rivers of hilly sub-Alpine area and even some waterways in Tertiary lithothamnium limestone areas. They are waiting for us to be discovered and appreciated.

Highland streams and rivers are swift because of geomorphologic features and the river regime marked by rain and snow.

However, the Sotla with its tributaries (except the Bistrica) presents a typical lowland river. The Sotla formed a valley bottom which is up to 1.5 m wide. Its slope is low and its meanders very frequent. They prevent effective draining and often cause floods. The largest flood areas are in its middle flow, around Podčetrtek, Imeno and Sedlarjevo. The Sotla's average flow is 9 m³/s at Rakovec. The river shed is extremely asymmetric; most tributaries join the Sotla from the right side.