GROUNDWATER BASICS - Interaction with Rivers or Streams
Rivers and streams interact with the local groundwater system either by losing water to the groundwater system ( a "losing" river) or gaining water from it (a "gaining" river). A single river can have some reaches (intervals along a river’s length) which are gaining and some which are losing.
Gaining river or stream
With a gaining river, the stage or altitude of the water surface in the river must be lower than the altitude of the water table near the river. The rate of seepage from the groundwater system into the river is proportional to the difference in altitude between the river stage and the nearby water table. In the Yuma area, the Colorado River is a gaining river from Laguna Dam to Morelos Dam. The U.S. Geological Survey made an extensive study of this portion of the Colorado River using data collected from 1975 through 1978, a period of normal river flow. They determined that during this period about 44,000 acre-feet/year of groundwater flowed into the Colorado River in this reach from the Arizona side and about 38,000 acre-feet/year from the California side. These figures should be representative of conditions for normal river flow today.
Losing river or stream
With a losing river, the stage or altitude of the water surface in the river must be higher than the altitude of the water table near the river. In a hydraulically-connected losing river, the river is in direct, continuous contact with the saturated zone. For such a losing river, the rate of seepage from the river into the groundwater system is proportional to the difference in altitude between the river stage and the nearby water table.
A hydraulically-disconnected losing river is one in which the river is separated from the saturated zone by an unsaturated zone. For a hydraulically-disconnected river, the rate of seepage from the river to the saturated zone through the unsaturated zone is independent of the difference in altitude between the river stage and the nearby water table. If the rate of seepage from the river and through the unsaturated zone is greater than the rate at which groundwater in the saturated zone can move away from the river, a ridge or mound of groundwater may develop below the river.
In the Yuma area, the southerly portion of the Colorado River from Morelos Dam to San Luis, Arizona, is generally a losing river. Probably much of it is hydraulically disconnected.
Change from gaining to losing
During a period of high flow, a river which is normally gaining may temporarily become losing until river stages (water surface elevations) recede. This occurred on portions of the Colorado River in the Yuma area during and after the flood of 1983.
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