While most of the population of the region speaks standard Polish, some people in the upper river basin (municipalities of Lipsk, Dąbrowa Białostocka and partly Sztabin) speak a local dialect of Belarusian (called by them prosty jazyk - "the simple language").
After the outbreak of the war, Olszyna-Wilczyński's unit was to prepare the defense of the area between Biebrza River, Suwałki, and Wizna.
The unit got surrounded by the Red Army in the Biebrza River area and suffered serious casualties, but managed to break through the enemy defences.
Near Mońki there are some of the greatest swamps in Europe, the Biebrza River Swamps, and in 1993 the government created the Biebrza National Park (BNP) to recognise and protect their unique nature.
The defenders were somewhat aided by fortifications from previous eras, including a major Imperial Russian-era fortress complex at Osowiec on the Biebrza River that was a scene of a siege in 1915 (February to August) during the First World War, and which was partly demolished by Wehrmacht troops in 1939 before its surrender to the Red Army.