The upland to lowland movement of groundwater continues if the water table under the hills is higher than the water level in, or under, the streams. Sections of streams that receive groundwater are called gaining streams (Figure 16a). As seen from the point of view of a rafter floating down a stream, gaining streams carry an increasing volume of water (becoming wider and/or deeper and/or flowing faster) with distance down the stream. This is the primary mechanism for groundwater to discharge to the surface. Water that enters a stream from the groundwater system sustains baseflow of the stream. In locations where the water table is deeper than the elevation of the water surface in the stream, water flows from the stream into the subsurface. Sections of streams where water seeps into the subsurface are called losing streams (Figure 16b and c). From a rafter’s viewpoint when floating down the stream, losing streams carry a decreasing volume of water, its discharge decreases with distance down the stream (becoming shallower and/or narrower and/or flowing slower) and may eventually become a dry streambed (Figure 16d). Streams that go dry during some periods, such as arroyos in the southwestern United States, are called intermittent or ephemeral streams. Streams that gain essentially all the time because there is enough recharge to store sufficient groundwater to support outflow to the stream throughout dry periods are called perennial streams.
Water flowing in streams is a combination of components contributed by:
- precipitation that flowed over land;
- infiltration that flowed through temporarily saturated soil zones above the water table (called interflow or quick flow);
- groundwater discharge; and,
- human activity (primarily flow from storm water sewers, point discharges from industries and water‑treatment plants, and agricultural drainage).
Groundwater discharge to streams is the primary reason that streams do not run dry despite days to months without rain. The flow continues because water recharged to the groundwater system long ago and far from the stream, flows slowly and steadily toward the stream.
Although the proportions of natural components of stream flow vary depending on local climate and geology, globally approximately half of the water flowing in rivers is from long‑term flow through the persistently saturated portion of the groundwater system discharged to the river, and half is from storm runoff over the surface or through shallow temporarily saturated layers. Some storm runoff occurs over the ground surface, though most storm runoff flows through the shallow subsurface, often initiated in the vadose zone via temporary saturated zones perched on low permeability layers. The zone of the capillary fringe near streams can provide a component of interflow when infiltration reaches the capillary fringe, changing the zone from a state of negative water pressure to positive pressure. In this case the water held in the capillary fringe is mobilized and the water moves rapidly to the stream. This phenomenon is known as the capillary fringe effect.
It is useful to note that some hydrologists refer to the flow in stream channels as runoff, but here we call that flow stream discharge while the term runoff is reserved for storm water making its way to streams either by flowing over the ground surface or through shallow temporality saturated soil layers.
A segment of a stream, such as shown in Figure 16, can be gaining on one day or in one season and losing on/in another, all depending on the relative water level between the stream and the surrounding water table. Conditions along a stream’s length may vary between gaining and losing many times. The water flowing at a location in the stream depends on two conditions. First, it depends on precipitation and/or snowmelt that occurred, perhaps many days ago, in upgradient portions of the area drained by the stream that flowed to and then along the stream channel. Second, it depends on the level of the water table directly below the stream. The water table below the stream may be high due to strong groundwater flow or it may be depressed due to drought or to heavy use by phreatophytes near the stream. Therefore, near the stream, the elevation of the stream water and the elevation of the water table can rise and fall for different reasons, and at different tempos, depending on both the local and distant weather, climate, vegetation, and terrain.
The continuous, dynamic, two‑way exchange between groundwater and surface water bodies illustrates the close connection between groundwater and surface water. At one moment a water molecule belongs to the groundwater reservoir, and at the next moment, it belongs to the surface water reservoir, only to return to the groundwater again. Appreciating this groundwater‑surface water continuity is important for managing our water resources.
Losing streams play an important role in recharging the groundwater reservoir in arid basins. In many arid regions of the world, the surrounding mountains receive more precipitation than the valleys, supplying water to the mountain streams. Upland stream beds are often higher than the water table. Where these streams flow out of the mountains and over sediments on the dry valley floor near the mountain front, they lose their water, thereby recharging the groundwater as shown in Figure 17. The lack of direct precipitation on the valley floor renders groundwater in such areas precious. The shallow water table in these dry valleys can result in gaining streams, as well as support plant and animal life that would otherwise be impossible as shown in Figure 17.
Humans in arid areas have tapped into this type of groundwater resource since ancient times by digging shafts into the mountains with long, low‑slope tunnels for conveying water to the desert plains and developing large, thriving agriculture‑based civilizations (Figure 18). Known as a Qanat or Kariz, these structures were first created about 3000 years ago in the Middle East.
In some places, humans divert water directly from streams, often for irrigation. When water is diverted from a losing stream, the human diversion deprives the groundwater system of recharge that it would have received from the stream.
Groundwater Moderation of Stream Temperature
The delayed arrival of groundwater recharge at streams provides a reliable source of water for the gaining streams of the world. This is important for the health of aquatic ecosystems, because fish and aquatic plants that form the base of the food chain for many animal species would not be able to live in a stream that runs dry as soon as the rain stops. The steady seepage of groundwater into streams also regulates stream temperature, because the groundwater is insulated from daily (and seasonal) heating and cooling of the atmosphere. Consequently, its temperature does not vary as much as the surface temperature as shown in Figure 19.
Thus, groundwater discharging to a stream is cooler than the stream water in summer and warmer in winter as illustrated in Figure 20. The warmer groundwater beneath the stream prevents freezing of the stream bottom. Again, this is important to fish and other aquatic life, many of which survive in only a narrow range of temperature. The groundwater seeping in through the bottom of a stream or lake may contain oxygen, and hence be favorable for fish to lay eggs, or the water may be devoid of oxygen (anoxic) and thus unfavorable for fish. Anoxic conditions may develop because of land‑use changes in recharge areas such as soil compaction, paving or disposal of organic wastes.