A decrease in the flow of the downstream stretch of the Białucha river (also known as Prądnik) has been determined based on the calculations of groundwater inflow to the Białucha riverbed completed for the Ojców‒Giebułtów‒ Olsza stretch using the Wundt and Kille methods. Hydrogeological analysis of this phenomenon revealed that the water is lost at the point where the Prądnik leaves the Proszowice Plateau and enters the Vistula Plain as the Białucha. The reason for the water outflow from the riverbed is the relatively low groundwater level in the alluvia of the Vistula ice-marginal valley. Upstream from this point, groundwater is supplied to the river in a narrow valley eroded in rocky ground, while downstream water flows out into a thick aquifer drained by the Vistula river that is situated much lower in this area. A similar phenomenon is observed at the mouths of most tributaries of the upper Vistula. This problem is discussed in detail in the master’s thesis by Iwona Podraza .