When substantial rainstorms come, and the ground is so saturated that it cannot absorb anymore, ground water and river levels can rise enough to create severe flooding.
In this picture, you can see the handrail that runs along the stairway going down, and it just disappears into the water. Due to massive amounts of rain over several days, the water level was 3-4 feet higher than normal.
It is especially important at these times to check basements and crawlspace areas to ensure sump pumps are functioning properly, because when water levels are this high outside, it doesn't take much for the water to come in.