What is "gjersjoeelva"

Gjersjøelva (former Hvitebjørnselva) is a river in the municipality of Oppegård in Akershus county, Norway. It flows from Gjersjøen and mouth into Bunnefjorden near Hvervenbukta, a total fall of . The last part of the river forms the border between Akershus and Oslo. Gjersjøelva had one of the first saw mills in Norway, from the 16th century.