ABSTRACT
Freshwater systems occur in terrestrial environments as a result of precipitation, water storage, and gravity. We can understand how freshwater systems form by following a drop of rain. The drop falls due to gravity, and that force also moves water downhill. Assuming our drop falls on a high mountain, it is most likely to strike the earth. If the soil is porous or dry, moisture may soak into the ground to be stored or evaporated. As water accumulates in porous soil, gravity will pull it deeper until it encounters a nonporous stratum in which it will accumulate, or if slanted, water may emerge as a spring or seep where the stratum is exposed. If the drop falls on a pond or a lake, it will be stored there for a time and will evaporate, seep into the ground to become groundwater, or be transported out of the lake with seasonal ¤ooding.
