Tyrifjorden (Lake Tyri) is Norway's fifth largest lake with an area of 137 km². It has a volume of 13 km³, 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.