Glomma, also known as Glåma is the longest and largest river in Norway, 598 kilometers long. Its drainage basin covers fully 13% of Norway's area, all in the Southern part of Norway.
At its fullest length, the river runs from lake Aursund near Røros in Sør-Trøndelag and runs into the Oslofjord at Fredrikstad. Major tributaries include the Vorma River, which drains Lake Mjøsa, joining the Glomma River at Nes. The Lågen drains into Lake Mjøsa, collecting drainage from the large Gudbrandsdal 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 hydro-power have been backed by excellent port facilities.
The upper limit of Glomma valley farms is variable, but typically runs about 500 meters (1650 feet) in the Østerdal, slightly lower than in the Gudbrandsdal, which reflects the colder climate. The treeline, with a light birch forest, rises to about 900 meters (3000 feet) in the Østerdal. Above Røros the forest is restricted to 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 Østerdal (or East Valley).
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 for in the construction of foundations in Norway.