INTRODUCTION: Streams interact with ground water in all types of landscapes (see Box B). The interaction takes place in three basic ways: streams gain water from inflow of ground water through the streambed (gaining stream) they lose water to ground water by outflow through the streambed (losing stream, Figure ), or they do both, gaining in some reaches and losing in other reaches. For ground water to discharge into a stream channel, the altitude of the water table in the vicinity of the stream must be higher than the altitude of the stream-water surface. Conversely, for surface water to seep to ground water, the altitude of the water table in the vicinity of the stream must be lower than the altitude of the stream-water surface. Contours of water-table elevation indicate gaining streams by pointing in an upstream direction and they indicate losing streams by pointing in a downstream direction in the immediate vicinity of the stream losing streams can be connected to the ground-water system by a continuous saturated zone or can be disconnected from the ground-water system by an unsaturated zone.

Where the stream is disconnected from the groundwater system by an unsaturated zone, the water table may have a discernible mound below the stream if the rate of recharge through the streambed and unsaturated zone is greater than the rate of lateral ground-water flow away from the water-table mound. An important feature of streams that are disconnected from ground water is that pumping of shallow ground water near the stream does not affect the flow of the stream near the pumped wells.

In some environments, stream flow gain or loss can persist; that is, a stream might always gain water from ground water, or it might always lose water to ground water. However, in other environments, flow direction can vary a great deal along a stream; some reaches receive ground water, and other reaches lose water to ground water. Furthermore, flow direction can change in very short timeframes as a result of individual storms causing focused recharge near the stream bank, temporary flood peaks moving down the channel, or transpiration of ground water by streamside vegetation A type of interaction between ground water and streams that takes place in nearly all streams at one time or another is a rapid rise in stream stage that causes water to move from the stream into the stream banks. This process, termed bank storage, usually is caused by storm precipitation, rapid snowmelt, or release of water.

From a reservoir upstream. As long as the rise in stage does not overtop the stream banks, most of the volume of stream water that enters the stream banks returns to the stream within a few days or weeks. The loss of stream water to bank storage and return of this water to the stream in a period of days or weeks tends to reduce flood peaks and later supplement stream flows. If the rise in stream stage is sufficient to overtop the banks and flood large areas of the land surface, widespread recharge to the water table can take place throughout the flooded area. In this case, the time it takes for the recharged floodwater to return to the stream by ground-water flow may be weeks, months, or years because the lengths of the groundwater flow paths are much longer than those resulting from local bank storage. Depending on the frequency, magnitude, and intensity of storms and on the related magnitude of increases in stream stage, some streams and adjacent shallow aquifers may be in a continuous readjustment from interactions related to bank storage and overbank Flooding. In addition to bank storage, other processes may affect the local exchange of water between streams and adjacent shallow aquifers. Changes in stream flow between gaining and losing conditions can also be caused by pumping ground water.

                                                                                                           

 

Previous Next