The Kasmala River watershed is situated in the southern forest-steppe subzone of the Ob Plateau (Altai Krai) and combines in its own structure the main geological-geomorphological elements of that territory. It is shown that this watershed can serve as a landscapehydrological model for all plateau’s rivers inheriting an ancient flow valleys. When comparing average monthly water discharge and annual hydrographs in 1942, 1945–1959, the area and zonal position of four river watersheds (Barnaulka, Kasmala, Kulunda, Burla) it became clear that the most part of the Ob Plateau river runoff is formed in its forest-steppe part. In the plateau steppe part precipitation is spent for evaporation and partly replenishment of ground waters therefore steppe fragments of the watersheds practically do not increase the annual runoff volume. Thus the study results of the model watershed runoff in the southern forest-steppe can be extrapolated to the Ob Plateau. It is revealed that the type of an annual hydrograph of the plateau rivers runoff depends mainly on a ratio of ridges and the ancient flow valley bottom in their watersheds. The higher the ridges share is the higher spring flood and lower low water are. Furthermore, the probability of the second small peak of discharge in October – November increases.