Le oscillazioni di livello del lago di Mezzano (Valentano - VT): variazioni climatiche e interventi antropici

Il lago di Mezzano é un lago di origine vulcanica presente all’interno della caldera di Latera (VT). Lo studio dei sedimenti e delle formedi origine lacustre, l’esecuzione di alcune trincee che hanno attraversato i terrazzi più bassi e la datazione di campioni col metodo delradiocarbonio, hanno permesso di riconoscere l’evoluzione recente del bacino idrografico del lago. In particolare é stato possibileindividuare varie oscillazioni di livello e cambiamenti di forma e di estensione del lago, indicate da notevoli variazioni nella distribuzionee nella quota dei sedimenti. La presenza di sedimenti databili alla porzione media e finale dell’Olocene ha permesso di ottenereinformazioni più dettagliate per gli ultimi millenni. L’individuazione di opere di bonifica antiche e recenti e la correlazione cronologicacon le oscillazioni di livello di altri laghi dell’Italia Centrale hanno permesso di stabilire che il livello del lago di Mezzano veniva influenzatoprevalentemente da fattori climatici, con l’eccezione degli ultimi 2000 anni, quando a tali fattori si sono aggiunti gli effetti dei tentatividi controllo del livello delle acque.

Lago di Mezzano is a lake of volcanic origin situated inside the Latera caldera. The surface area is about 0.5 km2, and the catchmentarea is about 1 km2 with few, very small, surface inflows and one outflow. Three submerged settlements of pile-dwellings and numerousmetal and pottery artifacts, attributable mainly to the Bronze Age, have been found near the lake shore. Study of the sedimentsand of the morphological features of lacustrine origin, the escavation of a number of trenches across the lowest terraces, and dating ofsamples by the radiocarbon method, have revealed the recent evolution of the lake’s drainage basin. The geological and geomorphologicalstudies demonstrate changes in the hydrological balance and in the drainage network discharging into the lake; these data furtherallows 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 considerablevariations in the distribution and depth of the sediments. The presence of sediments that can be dated from the Middle and LateHolocene has enabled more detailed information to be obtained for the last few millennia. The identification of ancient and recentreclamation works, and chronological correlation with the oscillations in level of other lakes in central Italy, have established that thelevel of lago di Mezzano was mainly influenced by climatic factors, with the exception of the last 2000 years, when the effects ofattempts to control the level of the water can be observed, in addition to said factors. Before the artificial drainage, when the level wasvery high and the lake larger than at present (e.g. during the XV – XVII centuries), the water reached the sill level forming an outflowtowards the Olpeta river. At periods of high water level the catchment to lake surface ratio was 2.6 calculated from the approximatecatchment 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 thestreams of the northern part of the catchment reached the lake in the southernmost place, flowing through the former lake bed.