The vertical infiltration of water is a variable saturated flow process until the soil under an inundated area is completely saturated. However, in several modelling studies that regard the river-aquifer interactions, the vertical infiltration under ponded conditions is modelled as a saturated soil flow process. In order to find out whether the assumption of saturated conditions under an inundation area is suitable or not, the saturated time of the soil below an inundated area is determined. This is realised by evaluating an analytical infiltration model with two example soils under different initial conditions and for different ponding depths. The model results show, that saturation of several meters of sandy soil under ponded conditions takes place within a period of hours. For the clay example, the saturation process happens within the order of days, and if the clay is initially dry, it can take even weeks. This leads to the conclusion that saturated model approaches are suitable for sand and gravel aquifers if they are not covered with a clay layer. While the ponding depth influences the infiltration rate and the progression of the saturation front in the sandy soil, for initially wet clay the infiltration rates were found to change only marginally with the ponding depth.