Aerial view of a river meandering through forested mountains with a small town alongside the river.
Smith River Aerial view of Smith River, California. © TNC
Groundwater 101

Rivers, Streams, and Estuaries

Most rivers begin high up in the mountains or hills, where winter snow melts and rain falls. Snowmelt and rainfall can also seep into the ground to form groundwater. Groundwater connects to rivers and streams on a permanent or intermittent basis. In gaining reaches, groundwater contributes flows to the stream. In losing reaches, groundwater is replenished by surface flows. Groundwater commonly contributes all or some of the water flowing down a river, particularly during a region's dry summer and fall months.

During periods of insufficient snowmelt or rainfall, or excessive groundwater pumping, groundwater levels can drop below the elevation of the riverbed, pulling surface flows away from rivers into the ground. Depending on the aquifer conditions, groundwater pumping farther away from the river can still impact groundwater levels below the riverbed. In some cases, sections of rivers and streams can go dry for longer periods of time. Since groundwater is often a source of critical cold water for rivers and streams during hot summer and fall months, habitat and cold water refugia for native fish can be lost.

In places where rivers and streams meet the ocean, estuaries can also depend on groundwater. Groundwater can be an important source of freshwater to estuaries, particularly in intermittent river systems or where upstream surface water use is substantial.