A more thorough explanation:

Definition: Undercurrent 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 and is also known as underflow of surface stream.

Examples:

• During dry seasons, the water in a stream may decrease, causing the undercurrent of surface stream to become more visible.
• Undercurrents of surface streams can also be found in areas where the stream flows over permeable rocks or soils.

These examples illustrate how the undercurrent of surface stream is a part of the overall flow of water in a stream. Even though it may not be visible on the surface, it still plays an important role in the movement of water and the overall health of the stream ecosystem.