The Tsiknias gorge is located a few kilometers North-East of the settlement of Arisvi and was formed by the erosion of the water, the river of the same name, which passes over the traces of large geological faults, with direction NW-SE and NE-SW.
Tsiknias is one of the largest intermittent streams of Lesvos. The stream drains a catchment area of about 90 km2 that occupies the southern slopes of the Lepetymnos volcano and includes most of northern and central Lesvos. The river bed has a general direction of flow NE-SW, while shortly before its mouth in the bay of Kalloni it turns to N-S.
The river flows exclusively volcanic formations from 21.5 to 17 million years old. When it meets the alluvial plains around Kalloni Bay, it flows Quaternary alluvial deposits, consisting of cobbles, sands and clays.
The valley of Tsiknias and the wetlands that form at its estuary are a habitat for birds, which live either seasonally or permanently along its bed, and mainly at its estuary. The river bed in the spring and when the waters calm down from the rains, is an important refuge for many migratory species: Common grebes, Sandpipers, Little herons, Porphyro herons, Nightingales, and also small birds such as ptarmigans, grebes, swallows, etc. Countless pairs of Nightingales nest here along with bee-eaters, Black-throated Shrikes, Common Pipits, Water Hens, while White Storks and Black Storks look for food throughout the summer. At the point of its estuary seabirds, waterfowl and waterfowl such as Mediterranean Gulls, Flamingos, Egrets, Cormorants, Sandwich tern, Common greenshank etc. take refuge all year round.