The Site covers a 35-kilometre section of a natural valley within the borders of the Narew River National Park in north-eastern Poland. The lowland river follows a sinuous path through the moraine hills of the wide valley with a well-developed system of meanders and oxbows. It is fringed with fens and reed and sedge beds interspersed with willow, alder and birch woods. Most of the swampy Site is inaccessible to people and provides excellent refuge for a rich flora and fauna, especially local and migrating waterbirds. It is one of the most important breeding sites in Poland for the aquatic warbler Acocephalus paludicola, and regularly hosts significant numbers of black-tailed godwit Limosa limosa. The main threats to the Site are the reduction in water input because of the upstream Siemianowka dam at the border with Belarus, and the water pollution caused by towns upstream. The water quality is however steadily improving because of progress in its treatment.