The hydrological regime of the Volga river near the Akhtuba river source below the dam of the Volga hydroelectric power station is studied. We use a hydrodynamic model based on the numerical solution of shallow water equations  to study the dynamics of discharge water during spring floods. The main result is the determination of the hydrological connection between the water discharge (hydrograph QV(t)) of the dam of the Volga HPP and the hydrograph of  the Akhtuba river source (QA(t)) depending on the dynamics of the water flow from the Volgograd reservoir to the Volga river. Numerical simulations of the spring flooding process were performed for the northern part of the Volga- Akhtuba floodplain for 2016, 2017 and 2021. The relationship between QV and QA is non-linear due to the peculiarities of the riverbeds and the interfluve topography. The results are important for developing solutions aimed at  preserving the Volga-Akhtuba floodplain.