What is "tyrifjorden"

Tyrifjorden (Lake Tyri) is Norway's fifth largest lake with an area of 137 km. It has a volume of 13 km, is 295 meters deep at its deepest, and lies 63 meters above sea level. The lake's primary source is the Begna river, which discharges into Tyrifjorden at Hønefoss where the river forms the waterfall of Hønefossen. Its primary outlet is at Vikersund near the lake's southwest corner, where Tyrifjorden discharges into the Drammenselva river.