The hydrogeologic characteristics of ground water are of primary influence as regards quantity and other distinguishing features of the water. In other words, regardless of relatively large quantities of percipitation, the water is not retained on the surface for any longer periods due to the ground's porosity.
Not so long ago, the inhabitants here supplied themselves with water from springs, wells, ponds, and cisterns which is still the case on the smaller islands. In a short description of the "sources" of water, we will try to indicate their importance. Let us begin with torrents. Torrents are swift-flowing streams which are caused by heavy rain and the configuration of the terrain. Shortly after the rain stops, these torrents disappear, leaving gullies behind. Near the settlement of Beli, during Roman times, one such gully had to be bridged. Another example is the gully on the southern side of Lake Vrana which may be seen from the main road, from the curve between the villages of Hrasta and Vrana.
In comparison to the inconsistency of torrents, springs and wells rarely dry up. During the rainy season water accumulates in underground caverns and lakes and is thereafter either emptied, which is the case with springs, or remains stagnant, in the case of wells. On the island of Cres alone, there are about 15 springs and 10 wells - e.g. at the southern end of the port of Cres (Piskel), south of Punta Kriza, near the village cemetery, (Funtuana) and other locations. As may be concluded, torrents, springs and wells are creations of nature. However, the ponds may either be natural or man-made. Even though most of today's ponds are unattended and are mainly used for watering livestock, their former purpose may be recognized by their form and appearance.
The ponds which were used for supplying the inhabitants, were mostly in the proximity of the settlements and can easily be identified by the fences that were built to protect the ponds from being approached by livestock. Such ponds were cleaned regularly. Ponds for livestock were situated on public or communal pastures ("komunada" - the old local name for them) or along the property limits of several pastures so that all the bordering owners could use them. Thus interesting solutions to the access of ponds were created, so called "diriti" (Ital. diritto:right). A third type of pond, according to its purpose, was the mixed pond. it was used both for the inhabitants and the livestock. The island of Cres has about seventy ponds.
Life today, particulary on the two largest islands, would not be possible were it not for Lake Vrana which has been a source of water supply to the populace for the past 40 years.
Lake Vrana is a cryptodepression with a total depth of around 74,5 meters and an average water level of around 13 meter above sea-level, so that its deepest end is 61,5m below sea level.
Because of it being so interesting aside from its importance, it has for a long time been the object of study. There have been various theories about its origin and subsistence. Today it is quite clear that Lake Vrana is an autochthonous lake, menaning that it is not connected with any sources on the mainland, as was once believed. Because of the form of the surrounding hills, water frows int it as through a funnel. An interesting fact is that even heavy rain does not have direct influence on the lake and is lost at its very bank. This leads to the conclusion that the lake is filled by underground currents. Due to the diversity of density of the porous ground, there are caverns or underground lakes somewhere under the lake itself which, when filled, let water escape into Lake Vrana. Since the lake exploited exclusively for supplying drinking water and has no significant source of contamination in its vicinity, the water is of supreme quality and is classed among the purest in Europe. For ther´se reasons access to the lake is most strictly forbidden. Do not brak this rule because, besides having to pay a very heavy fine, you will be endangering a part of nature that is still unpolluted.