The USGS Water Science School
My driveway with some water on it... what could this have to do with ground-water seepage? This picture really demonstrates why there is water in many streams even after long periods of no rainfall. You have to "look below the surface" (pun intended) of what you see here. The water on the driveway is not runoff from the surface, but is seepage out of the ground after a downpour the day before.
During the rainstorm water soaked into the ground in the hill above the driveway. As happens with water below ground, it started moving along underground layers of soil and rock that are porous enough to allow water to move through it. After a storm, water doesn't move straight down into the ground, but, rather, it moves both downward and horizontally along permeable layers. The water is moving downhill ("down-gradient") toward a creek at the bottom of the hill.
Normally, the water would just flow underground to the bottom of the hill and seep out of the stream banks into the creek. But here the driveway was dug deep enough into the ground so that it cut into the permeable layer of soil that carries the underground water downhill. Thus, you can see ground-water seepage coming to the surface.
By the way, it is seepage such as this that helps keep water flowing in many creeks and streams during periods of drought.
Ground water Water flowing from a road cut