The dynamics of groundwater movement have an important effect on stream flow. Groundwater that migrates into the stream channel increases stream flow; water in a stream can also enter the unsaturated zone, reducing stream flow.
Gaining streams. A gaining stream (Figure 1) is one into which groundwater flows from the saturated zone. The channels of gaining streams are usually at or below the level of the water table. Bodies of water and marshes form when the water table intersects the land surface over a broad, fairly flat area.
A Gaining Stream
Losing streams. The channel of a losing stream (Figure 2) lies above the water table and loses water into the unsaturated zone through which it is flowing This water then migrates down toward the water table. A losing stream can induce the local water table to rise. In drier climates a losing stream may actually disappear underground as its water content becomes progressively diminished downstream.
A Losing Stream
Springs. A spring is a natural flow of groundwater from a rock opening that results when the water table intersects a sloping land surface. Springs can be seasonal—for example, during the wet season the saturated zone is closer to the surface because of increased rainfall, often resulting in more springs.
Aquifers. Aquifers are porous, permeable, saturated formations of rock or soil that transmit groundwater easily. The best aquifers are coarse‐grained sediments such as sand and gravel. A confined aquifer is overlain by a less permeable bed that keeps the water in the aquifer under pressure; an unconfined aquifer does not have a confining bed that separates the zone of saturation from the unsaturated units above it. Impermeable formations such as shale, clay, or unfractured igneous rocks that retard water flow are called aquitards or aquicludes.
If more water is removed from an aquifer through pumping than is introduced through recharge, the water table drops. This often results in wells that go dry or a surface that sinks because the ground surface is no longer as supported. This subsidence inflicts expensive damage on buildings, roads, and pipelines. Heavy use of an aquifer can be balanced through artificial recharge, a process by which treated industrial wastewater or floodwaters are stored in infiltration ponds. The water soaks into the ground to replenish the groundwater or is pumped back into the aquifer.
Wells. Wells are drilled into the water table to tap aquifers for domestic, industrial, and agricultural use. The level of the water table fluctuates with changing climatic conditions. During a dry period, the water table drops to a deeper level because water has drained out of the saturated zone into the rivers. During wet periods the water table rises because of the additional water percolating down from the surface into the zone of saturation.
The water table tends to be closer to the surface in valleys than on hillsides. Recharge occurs in those areas where new water is added to the saturated zone and replenishes water that has been lost.
The best wells are drilled deeply enough to supply a continuous flow of water during all the seasonal changes in the water table—thus they draw water from below the lowest level of the water table. In artesian wells, which tap water from confined aquifers, either the water level in the well simply rises above the aquifer (nonflowing artesian wells) or it spouts at the surface (flowing artesian wells). Whether a well is flowing or nonflowing depends on the amount of pressure that is exerted on the groundwater in the confined aquifer.
When water is pumped from a well, the water table is generally lowered around the well. This local lowering of the water table is called drawdown. Centered on the stem of the well, it has the shape of an inverted cone called the cone of depression. The drawdown decreases with increasing distance from the well.