What defines a losing stream?

A losing stream is characterized by the process where water flows out of the stream and into the surrounding groundwater system. This typically occurs when the water table is lower than the stream bed, causing the stream to lose flow. As the water seeps into the ground, it contributes to the replenishment of the groundwater, which can be crucial for maintaining local aquifers.

In contrast, streams that receive additional water from tributaries or maintain constant water levels do not exhibit the same losing characteristics. Additionally, while a stream that flows through an underground aquifer may have interactions with groundwater, it doesn't specifically denote a losing stream unless water is actively leaving the stream into the aquifer. Thus, the defining aspect of a losing stream is its tendency to lose water to the groundwater system, which is what makes the selected answer accurate.