Sourpi has a natural wetland recognized by the International Ramsar Treaty, with cones, willows and plane trees spread out over 1,100 square metres. It is a marsh with salty water where herons, nightingales and other kinds of birds spend the winter. The River Salambrias was the border of the first Greek state after the revolution, from 1830 to 1881. You can hike the path of Vathyolakos from Soupri to the top of Ai-Lias.
This combined marshy and marine environment is supplied with fresh water from a well and a stream while the seawater that enters the marsh renders the water salty. Rare species of migratory and non-migratory birds spend their winter, including swans, herons, etc. Some reproductive species include the Bittern, Little Bittern and Rufous Bush Robin.