A more thorough explanation:

Definition: Underflow of surface stream refers to the slow movement of water through the bed of a stream or the lands under or immediately adjacent to the stream. This water is considered part of the surface stream.

Example: When a river flows over a permeable rock layer, some of the water seeps through the rock and moves slowly underground. This water is still part of the surface stream, but it is not visible on the surface. This is an example of underflow of surface stream.

Explanation: Underflow of surface stream occurs when water moves slowly through the bed of a stream or the lands adjacent to the stream. This water is still considered part of the surface stream, even though it is not visible on the surface. The example of water seeping through a permeable rock layer illustrates how underflow of surface stream can occur.