, during winter , a major
tributary, flows into the Glomma just north of
Årnes At its fullest length, the river runs from the lake
Aursunden in
Røros Municipality in
Trøndelag county and runs into the
Oslofjord at the town of
Fredrikstad in
Fredrikstad Municipality in
Østfold county. Major tributaries include the
Vorma River, which drains
Lake Mjøsa, joining the Glomma River at
Årnes in
Nes Municipality. The large river
Lågen flows into Lake Mjøsa, draining the large
Gudbrandsdalen valley and significantly increasing the Glomma's flow. Because it flows through some of the richest forest districts, it has historically been Norway's leading log-floating river. The combination of raw materials, water power, and easy transport has over the centuries encouraged industry along the Glomma. Some of the country's largest manufacturing and processing concerns are found around its mouth, where supplies of timber and
hydropower have been backed by excellent port facilities. The upper limit of the
Glomma valley farms is variable, but typically runs about in
Østerdalen, slightly lower than in the Gudbrandsdalen, which reflects the colder climate. The treeline, with a light birch forest, rises to about in Østerdalen. North of
Røros, the forested areas are only on the valley floor. The upper river valleys of Norwegian rivers have distinctive names which are vestiges of earlier cultural distinctions such as building styles, traditional clothing or
bunad and domestic crafts. The upper valley of the Glomma is the
Østerdalen (). Upon entering
Lake Øyeren at
Fetsund, the Glomma has formed Europe's largest inland delta which reaches the opposite side of the lake, across its short axis. Some of the vast amount of
silt that the Glomma deposits in Lake Øyeren is extracted to manufacture
LECA building blocks widely used in the construction of foundations in Norway. ==Name==