The Biebrza Marshes fill the broad valley of the Biebrza River in Podlaskie Voivodeship, north-eastern Poland, in the 130 km-long Biebrza Basin north-west of Białystok. They are the largest marshland in Poland and one of Europe's largest surviving lowland fens: open sedge fen, reed beds, wet meadows and alder and birch woods on peat that is in places more than ten metres deep. Most of the valley is protected by Biebrza National Park. Created on 9 September 1993, it is Poland's largest national park at 592.23 km², and it has been a Ramsar wetland of international importance since 1995. Life in the marshes follows a yearly water cycle. Winter freezes the fen, spring snowmelt floods the valley floor, summer brings drought and mowing, and autumn draws the river back into its channel.
A 2,600 km² Basin Built to Hold Meltwater
The marshes owe their seasons to the shape of the valley. The Biebrza Basin (Kotlina Biebrzańska) is a lowland depression about 130 km long, up to 35 km wide and roughly 2,600 km² in area. During the last, Vistulian glaciation it was an ice-marginal valley that carried glacial meltwater south to the Narew. When that flow ended in late-glacial and Holocene times, the waterlogged floor began to fill with peat. The basin floor drops only from about 120 m in the north to 98 m where the Biebrza meets the Narew near Wizna. It falls into upper, middle and lower basins, with a narrow neck at Osowiec-Twierdza between the middle and lower ones.
The river itself is about 165 km long and drains a catchment of roughly 7,050 km². About 155 km of it flows inside the national park. The catchment is lopsided: three-quarters of it lies on the right bank, fed by the Netta, Ełk and Wissa coming down from the Masurian lakeland, while the Sidra and Brzozówka join from the left. The Biebrza's average flow of 34.9 m³/s is greater than the Narew's 32.5 m³/s at Strękowa Góra, just above the confluence. Polish sources credit its valley with the largest natural water-storage capacity of any in the country, comparable to Poland's biggest reservoirs. That storage matters most in spring.
December to February: Ice on Czerwone Bagno
Winters in north-eastern Poland run several degrees colder than in most of the country. Białystok, the regional capital, averages just 7.7 °C over the year and has Poland's shortest growing season, about 205 days. Cold air from Siberia or the Arctic arrives here more often than in the rest of Poland, frosts below −20 °C are possible though rare, and snow lies on the ground for more than half the winter. In the marshes this means frozen fen and ice-bound river channels. The wet woods and bogs of Czerwone Bagno (the "Red Marsh") freeze over with the rest of the valley. This strict reserve was first protected between the world wars and is still the most valuable core of the park.
Winter is the best time to see what the marshes are famous for, the moose (elk in British usage). Biebrza is the largest moose refuge in Poland, with about 400 animals. The population descends from a few moose that survived the Second World War in Czerwone Bagno and were then helped to recover. Beavers are also back: today's Biebrza beavers descend from animals brought to Osowiec from Voronezh in Russia in 1945. That fits the river's name, which probably comes from a Baltic word for beaver (Lithuanian bebras). The name appears in a 1358 document of Duke Kęstutis describing the border with Mazovia. Wolves are seen only rarely.
March and April: The Snowmelt Rozlewiska
When the snow melts, the Biebrza overflows. Water spreads across the valley floor in shallow sheets the Poles call rozlewiska, and the lower and middle basins become temporary lakes broken by islands of higher ground. In the southern basin, below Osowiec, those dry islands are called grądy, and many have their own names: Kaliszek, Koźli Rynek, Kobielne, Pogorzałe, Kiermuz and Lisie Jamy. Around them lie well-watered peatlands with many oxbow lakes, and the wild Ławki marsh. Because the valley can hold so much water, the flood fills the Biebrza Basin first and only slowly passes on down the Narew towards the Vistula.
The spring flood is also the main season for visitors. A marked 135 km water trail, with campsites and viewpoints, runs down the river. Kayak and raft trips are most popular during the spring high water, when boats can float over meadows that are dry in July. Since the 2002 classification of inland waterways, the Biebrza has been a class Ia waterway. Through the Netta and the Augustów Canal, built between 1823 and 1839, it links the Vistula basin to the Neman.
April Fires: 2020 and 2025
In a normal year the flood protects the valley from fire. When it fails, dry sedge and reed from the previous summer go up quickly. On 18 April 2020, when the spring flood had failed to arrive, a fire started by illegal grass burning swept through the park. With the water table low, the fire burned until 26 April. Estimates of the damage range from about 5,280 hectares, roughly 9.5% of the park, to at least 6,000 hectares. It was the worst fire since 1992. Nearly 2,000 firefighters, soldiers and rescuers fought it with six firefighting aircraft and two helicopters, which made close to a thousand water drops. The state budget paid almost 8 million złoty for the operation. Investigators concluded the fire had been set deliberately, but the prosecutor closed the case without identifying anyone. Surveys afterwards found significant declines in 14 bird species, the aquatic warbler among them.
Fire returned on 14 April 2025, with the valley again short of water. About 500 people and five helicopters fought it until 25 April. It burned about 450 hectares, mostly sedge and reed beds. Only about 22 hectares of Czerwone Bagno were lost, 0.5% of the reserve.
May and June: Ruff Leks and Snipe Displays
As the floodwater drains away in late spring, the wet meadows become one of the most important wader breeding grounds in Central Europe. The park counts 271 bird species, and since 2010 its whole area has been listed by BirdLife International as an Important Bird Area. Its emblem is the ruff (batalion in Polish). In the May–June breeding season the males grow coloured head tufts and neck ruffs and display to females at communal leks. The great snipe also displays on leks at dusk, with rattles, clicks and whistles. Its European breeding population is in steep decline, which makes the great snipe, one of the park’s characteristic breeding birds, all the more valuable here. Common snipe, Eurasian curlew, dunlin, common crane, white-winged and whiskered terns, lesser spotted eagle, eagle owl and short-eared owl also breed in the valley.
The wildlife does not stop at birds. The park lists 49 mammal species, 36 fish, 12 amphibians and 5 reptiles, plus more than 700 species of butterflies and moths and 448 spiders. Pike, catfish and eels live in the river. The fens hold orchids, sundews and cold-climate relics such as the dwarf birch and the downy willow, plants that are disappearing elsewhere in Poland.
July to October: Hay, Low Water and the Aquatic Warbler
By midsummer the valley is at its driest, and how it is managed decides which birds come back the next year. The aquatic warbler is the rarest songbird in mainland Europe and the only globally threatened passerine there, with an estimated 11,000–15,000 pairs. It nests only in wet sedge beds where the vegetation is shorter than 30 cm. Its main threats are drainage, the decline of traditional extensive farming, and the spread of reeds, shrubs and trees over abandoned meadows. On fens like Biebrza’s, keeping the meadows open, so that reeds and willows do not close in, matters as much as keeping them wet. After the breeding season the warblers fly south-west to winter in West Africa, and many of them gather at Djoudj National Bird Sanctuary in Senegal.
The dry months are also when people move most easily through the valley. The 33 km stretch of the Carska Droga ("Tsar's Road") runs along embankments and dunes through the marshes, from the Narew bridge at Strękowa Góra to Osowiec-Twierdza, and is known for encounters with moose. It was built around 1900 to link the Russian fortresses of Łomża, Osowiec and Grodno. Osowiec Fortress itself was built between 1882 and 1892 at the one point where the Biebrza marshes could be crossed. It held out against German attacks from September 1914 until the Russian army abandoned it in August 1915. Today it houses the park headquarters. In the upper basin around Lipsk and Sztabin, some villagers still speak a local Belarusian dialect they call prosty jazyk, and a few Old Believer villages on the north bank use an archaic Russian dialect. Plans would add about 8,400 hectares to the park.
Biebrza's floodwater ends up in a much larger river system, which you can follow in our guide to the Vistula. For a comparison on a far bigger scale, see the Pripyat Marshes of Belarus and Ukraine, part of the Polesie region where, together with eastern Poland, about 70% of the world’s aquatic warblers breed. To tell a fen from a raised bog, read what makes a bog a bog.