Earth

Springs

Ground water coming out on the surface is called a spring, if the origin of the water is unknown, or if it comes from an unconfined catchment area. It is called a resurgence if it is on the contrary the re-emergence of a watercourse that sank upstream, as the already mentioned case of the Timavo river. Springs can be classified in various ways, depending on the flow rate, the constancy of the discharge, or the geological characteristics that determine its formation. Many springs are characterized by a perennial flow, even though there can be remarkable differences in the discharge, depending on precipitation. Other springs, as the already mentioned overflow springs, may be characterized by a temporary outflow. From the name of the famous Fontaine de Vaucluse spring (France), springs characterized by a vertical development and a great depth (in many cases over 300 m) are called vauclusian springs. Thanks to the particular shape of the outlet galleries, some may have an intermittent outflow. A regularly intermittent outflow may also be observed in springs near the sea, where the influence of tides may be felt. The direction of the current is generally from inside the cave outward, and generally remains constant, but in some cases the flow may be inverted. Some springs in fact can alternatively act as springs or sinkoles. This depends on precipitations with a seasonal variation, as in the case of the ponor described before, or near the coast, due to the tide effect, as in the katavothre (for example those in the island of Cephalonia), or the submarine springs also known as estavelles. In some particular types of submarine springs, the vrulja, the flow rate of the fresh water feeding springs through karst conduits is such that it contrasts the pressure of the sea water, and in some points it exceeds it, thus giving rise to bubbling waters, a phenomenon that is well known along the Greek and Dalmatian coastlines, and once upon a time was exploited by sailors to collect a supply of drinking water. The speed of flow in these springs can be impressive, so much so that in the past they have often given rise to legends about sea monsters.