Location
The mouth of the biggest Polish river is located in the southernmost part of the Gulf of Gdansk, east of Gdansk, between two villages – Swibno and Mikoszewo.

Biggest value
The living estuary of a big river is characterized by dynamic changes in the arrangement of sandy mounds, miniature islands and peninsulas. It is an important breeding area for birds living on beaches and dunes, important feeding area for migrant birds (waders, gulls and ducks) as well as an important wintering site for birds of north-east Europe and Asia (ducks and sawbills).

Natural history
The formation  of the mouth of Vistula’s main current continues since 1895, when the canal was built to direct the river northward. Earlier Vistula’s estuary to the Baltic Sea was located in Gdansk (now the mouth of Martwa Wisla is located there) and in the winter of 1840 during a flood a new estuary was formed in Gorki (now the mouth of Wisla Smiala). The natural process of forming the Vistula’s estuary started over a hundred years ago and continues till this day. The sediments carried by the river form sandy islands and peninsulas, which  dynamically change their shapes.

At first the sediments were deposited in front of the river’s mouth and there was a danger of clogging up Vistula’s new estuary. Thus, up to the 1930s Vistula’s river bed was systematically deepened. At the end of  the 1960s a small Mikoszewskie lake was formed as the result of closing a narrow sand-bar to the right of the estuary.

Ecosystem
The ecosystem of the mouth of Vistula river is rich and attractive, mainly for birds. The main elements of the ecosystem are: the river, the narrow waters of the Gulf of Gdansk, Mikoszewskie Lake, the beaches, white and grey dunes as well as the nearby thickets, forests and meadows located alongside the river banks south of Swibno and Mikoszewo (flooded in spring and pastured in summer and autumn).

The area where the river flows into the sea, with the narrow waters of the river (carrying a multitude of nutrients) is an ideal place for aquatic invertebrates. This makes Vistula’s  mouth a feeding area for waterfowl, waders and those dependent on beach habitat.

Over 250 bird species were observed in the mouth of Vistula river and in its vicinity, 61 of them are breeding species, and over 40 are rare or very rare in Poland. The number of terns (common tern, little tern, sandwich tern) nesting on beaches and dunes decreased dramatically due to the activities of predators and humans. In summer three are little gulls (up to 40,000 birds). From middle July till the end of September numerous migrant waders stop at Vistula’s mouth (27 species – with dunlin, little stint, knot, curlew sandpiper and grey plover as the most numerous group. In winter there are about 60 bird species in the area, over half of them are waterfowl and waders.

This area is very important for long-tailed duck (up to 30,000 birds), tufted duck, goldeneye, scaup, goosander and mallard. Such a great number of waterfowl attracts white-tailed eagles – at least few of them winter in the area. The mouth of Vistula river is one of a few places in Poland where snow bunting winter regularly (up to 140 birds). However the most numerous species in the area is common gull (up to 100,000 birds).

Threats
Among the main factors, which threaten the natural processes of the formation of the river’s mouth, as well as animals living in the area are:

• Wloclawek reservoir on Vistula river, where sediments carried by the river – essential for the natural process of the estuary’s creation - clog up;
• frightening off birds by people penetrating the region of Vistula’s estuary during breeding season and migration season;
• predators (hooded crow, herring gull, fox, raccoon, American mink, cat) destroying nests and frightening off rare birds;
• willows, planted on the dunes in order to strengthen them, overgrow the breeding areas of terns;
• birds getting caught in the fishing nets;
• starting fires in the thickets on the dunes;
• pollution.

Nature protection
In 1991 the nature reserve "Mewia Lacha" (131,5 ha) was established in the mouth of Vistula river. The main aim of the reserve is to protect the newly shaped sandy parts of the mouth, where terns and plovers breed regularly. From 15 April until 15 August, in the nesting and breeding seasons, the entry to the reserve is forbidden. Moreover, the Sobieszewo Island located on the left bank of the river constitutes the area of protected landscape.