Change in surface / ground water interaction

An alteration in the linkage between surface flow in a river and underlying groundwater. This interaction occurs in the hyporheic zone, with flows transferring into and out of the bed and banks of the channel. The importance and extent of this interaction is dependent on a range of factors, including substrate type and underlying geology. For example, surface-groundwater interaction in a gravel-bedded chalk stream is likely to play a significant role in controlling water levels. Conversely, this interaction is likely to be less important in a relatively impermeable clay river system. In addition to controlling water levels in some river systems, the zone in which surface and groundwater interactions take place provides important habitats for benthic and interstitial organisms, rooting plants and spawning fish. Any changes in the interaction between surface and groundwaters can therefore disrupt these habitats.