Base flow refers to the surrounding ground water that seeps into the banks of a river or riverbed over time. Without base flow, many rivers and streams would only carry a flow of water during rainfall or storms.
When water from the ground increases water flow in a stream, it is referred to as a gaining stream. This occurs when the ground water level is higher than the water level in the stream.
It is also possible for streams to leak water to the surrounding ground. This occurs when the water level in a stream is higher than the water level in the ground and is referred to as a losing stream.