Picture of the Middle Brook, a tributary of the Raritan that flows through Bound Brook
Data retrieved from the National Weather Service in Mount Holly, NJ
Because of the rain falling so heavily in such a short amount of time, the ground was unable to soak up the water fast enough. Thus the water ran off towards lower elevations and eventually into streams, creeks, brooks, and rivers. The rivers (though relatively low after the summer drought) quickly filled with water and some eventually overflowed in great amounts.
The Raritan River was one of the many flooded waterways in the state. Running through the central portion of the state and spilling into the Raritan Bay near Perth Amboy, the raging flood waters inundated many low lying communities that were located within the flood plain. (Click on the picture for a closer look.)