The borders of the area where military battles of the winter Masuria-Augustów operation of February 1915 took place were designated by the rivers and lakes. In the west - by the Wegorapa River and the course of Masurian lakes, in the north and east - by the Neman River, in the south - by the Biebrza and Narew Rivers. The Germans occupied Gumbinenn district whereas the Russians - a part of the Suwalki Province, majority of Hrodna Poviat in Hrodna Province and a small part of Kaunas Poviat in Kaunas Province. In this operation the following natural obstacles were of the greatest importance: 1. the course of Masurian lakes extending along the Wegorapa River and further to the north the woodland stretching to the Szeszupa and Neman Rivers. 2. Elk group of lakes extending south-east to Rajgród. 3. Augustów primeval forest with its lakes and swampy areas. 4. the Biebrza River with its swampy valley. The activity of the Scandinavian continental glacier as well as climate factors had impact on the surface features of the Masuria Lake District. Water from melting glacier was running south and furrowed deep valleys where large lakes were formed. The course of Masurian lakes played an extremely important role during the warfare in East Prussia. It separated the belligerents along the line of 80 km from Angerburg (Wegorzewo) to Johannisburg (Pisz). The military importance of the lakes was increasing because they were situated on a hilly terrain. A group of lakes surrounding Augustów is generally called Augustów lakes. Sixteen out of them located near the town is over 1 square km in size. In the Masuria-Augustów operation the Rivers: Neman, Biebrza, Narew and Wegorapa, played an important role. Three primeval forests from the woodland of East Prussia and northern Podlasie were of significant importance, i.e., Borecka, Pisz (965 aquare km) and Augustów primeval forests. The first two, together with the Masurian course of lakes, were a difficult barrier to cross. They forced the attacking enemy to make a wide circular movement both from the north and south. The climate of the warfare area was mild but subject to sudden changes. Rapid transitions from thaw to hard frost with blizzards and snowstorms occurred quite often. Both parties experienced climate's instability in February 1915. Railway was a reliable means of transport. The Germans had the best developed railway network among European powers before the World War I and the East Prussia theater of warfare was crossed by numerous railways. The official German statistics from 1910, which can be hardly recognized as benevolently made in Poland's favor, specifies that 80% of the rural population in Masuria were Poles whereas, generally, Polish population in East Prussia was estimated to be over 500 thousand.