Construction of Mountain Flood Control Reservoir is an effective method of flood protection on mountain rivers. The Flood Control Reservoir works under the extreme condition of rapid water level rising and falling. The regime of intensive water level change affects the floodplain stability. Simulation of water penetration into the depth of soil, formation of the seepage curve under transient conditions during water rising and falling was carried out by Midas GTS NX software. A slope stability analysis was conducted for the floodplain of the Irshava River, flood of 1% probability (return period of 100 years) was assumed. Numerous simulations showed that the influence of the mechanical and hydraulic properties of the soil depends on the slope angle. The stability of the floodplain slope depends on the cohesion and hydraulic conductivity for steep slopes greater than 26°; if the slope is less than 26°, the stability is considerably dependent on cohesion, and the hydraulic conductivity practically has no influence. Thus, floodplain with the angle greater than 26°, additional geological surveys are required to determine the soil composition, mechanical and hydraulic properties for each layer. For the slopes lesser than 26° it is sufficient to determine the mechanical and hydraulic properties for the engineering and geological elements of soil, according to the national building regulations. Numerical simulation allows reducing the total volume of geological survey by minimizing survey of the slopes with an angle of less than 26°, which is 30% of the floodplain perimeter. The slope stability analysis showed that the slopes with the angle greater than 26° require stabilization structures in the lower part of floodplain before the Mountain Flood Control Reservoir construction.