LE OSCILLAZIONI DI LIVELLO DEL LAGO DI MEZZANO (VALENTANO - VT): VARIAZIONI CLIMATICHE E INTERVENTI ANTROPICI

Abstract

Giraudi C., The Lago di Mezzano (Valentano – Viterbo) level oscillations:climatic variations and human impact. (IT ISSN
0394-3356, 2004).
Lago di Mezzano is a lake of volcanic origin situated inside the Latera caldera. The surface area is about 0.5 km2
, and the catchment area is about 1 km2 with few, very small, surface inflows and one outflow. Three submerged settlements of pile-dwellings and numerous metal and pottery artifacts, attributable mainly to the Bronze Age, have been found near the lake shore. Study of the sediments and of the morphological features of lacustrine origin, the escavation of a number of trenches across the lowest terraces, and dating of samples by the radiocarbon method, have revealed the recent evolution of the lake’s drainage basin. The geological and geomorphological studies demonstrate changes in the hydrological balance and in the drainage network discharging into the lake; these data further allows the lake level variations evidenced by archaeological settlements to be framed in a clear palaeogeographic context.
Various late-Pleistocene and Holocene oscillations in level and changes in the lake’s shape and extent have been indicated by considerable variations in the distribution and depth of the sediments. The presence of sediments that can be dated from the Middle and Late Holocene has enabled more detailed information to be obtained for the last few millennia. The identification of ancient and recent reclamation works, and chronological correlation with the oscillations in level of other lakes in central Italy, have established that the level of lago di Mezzano was mainly influenced by climatic factors, with the exception of the last 2000 years, when the effects of attempts to control the level of the water can be observed, in addition to said factors. Before the artificial drainage, when the level was very high and the lake larger than at present (e.g. during the XV – XVII centuries), the water reached the sill level forming an outflow towards the Olpeta river. At periods of high water level the catchment to lake surface ratio was 2.6 calculated from the approximate catchment size of 3.57 km2 and a lake area of about 1.36 km2. When the lake level was very low, the basin became endorheic and the streams of the northern part of the catchment reached the lake in the southernmost place, flowing through the former lake bed.