This figure depicts overall ground-water flow from left to right. Individual water molecules follow a tortuous path in going from left to right. This constant change in direction that individual water molecules must undergo in traversing a distance results in much mixing of the ground water along with any contaminants that may be dissolved in the water. Along with diffusion, mechanical mixing is one of two processes responsible for dispersion of contaminants in the water table . The answer to this can be illustrated by the figure below.
Water is lost from the overlying stream.
This water must flow between individual soil particles, or more precisely, the water must flow through the soil pores. Just to reiterate, the soil pores are analogous to water pipes. Then, the water drains to the basement. Obviously the water that is lost from the overlying stream bed, drains vertically-downward through the soil to recharge the water table below. Typically, water flows from a higher elevation to a lower elevation, because of gravity.