Earth

Base level

The peculiarity of the base level is that it is not a fixed unchanging level, but it varies in (geological) time. It generally tends to lower progressively, as valleys deepen getting more and more entrenched. When a valley cuts the phreatic zone of a karst system the cave conduits that were previuosly filled with water get empty, letting water drain out. Thus karst springs are formed. These are normally found close to the base level, on valley floors – at times directly feeding water courses, at times creating pocket valleys. A subsequent deepening of valleys leads to the formation of springs at a lower level and to the fossilization of the oldest springs, which remain perched over the valley floor, and above the new base level. During exceptional flooding events, when springs at the base level are unable to take away the large discharge of water flowing through them, the karst water table inside caves may rise, invading the upper galleries, which are normally inactive, thus temporarily flowing out of the ancient springs: these are known as overflow springs. Also in this case, a good knowledge of the behaviour of the karst systems and the geological structure are very important in order to be able to foresee possible hydro-geological problems. The new activation of ancient galleries and springs, in fact, is often sudden and difficult to foresee, if one does not know the structure and the behaviour of the karst systems.