The Regional Park Valle del Treja, established in 1982, covers a territory of 650 hectares located in the middle stretch of the river Treja, right tributary of the Tiber. Its territory lies in the municipalities of Mazzano Romano and Calcata, respectively in the Province of Rome and Viterbo.
The Park is strongly characterized by a landscape of gorges, vertical walls carved in volcanic rocks from the river flowing, surrounded by rolling hills planted with arable crops, orchards, vineyards, olive groves and hazelnut. The river, at points of greater consistency of the rocks in which it is gouged, is interrupted by picturesque waterfalls, large and small, among which those of Montegelato, at which there is an old water mill. The area of the park is covered for more than half by forests, mostly in public ownership. The most common form of vegetation is of mixed forests of oak, like “cerro” and “roverella”, along with maple, hornbeam and hazel. Along the river prevail hygrophilous species, namely loving water, such as willows, poplars and alders, while the edge of the ravines are dominated by the more thermophilic species, those who like a warm environment, such as oaks and hackberry.
In the "heart" of the Park is an important archaeological site. The oldest archaeological findings in the area date back to the Bronze Age (1440-1200 BC). The great extension of the many necropolis (IX-VIII century BC to the IV-III century BC) testifies the presence of a center of significant size and importance, whose oldest part was located at the foot of the hill of Narce. The numerous objects found in burial sites scattered throughout the park and its immediate vicinity witness an ancient and vibrant culture. Currently most of the objects recovered in the numerous excavations carried out between the end of the 19th century and the first decades of the last century are on display at the Archaeological Museum of Civita Castellana, and at the National Museum of Villa Giulia in Rome. Even today in the park you can admire the remains of several tombs, of roads, of tunnels constructed for hydraulic purpose, and fortifications erected to protect the ancient town of Civita Castellana, famous for being the capital of Falisci population in pre-Roman times. At the foot of Mount Li Santi, a few meters from the right bank of Treja River, are the ruins of an old monumental building, probably used for worship of female fertility.
The Park Valle del Treja can be visited all year round, the route along the river, with its windmills and waterfalls, is particularly evocative and full of environmental significance, but also full of sights and landscapes of rare beauty. The small towns of Mazzano Romano and Calcata are famous for their characteristic positions, for the narrow streets of their old town centers, and for the very special landscapes.