Lake Porkuni (area 44.6 ha, length 2 km) is situated in the old valley of Porkuni, 107 metres a.s.l. The water-body is mostly formed by dredging and impounding. This is done several times, first when the bishops castle was founded and later in the manor period. The lake consists of four parts.
Lake Porkuni, that is fed by several springs’ water, is the headspring of the River Valgejõgi. The springs are situated in the Southern end of the lake. The River Valgejõgi (length 85 km) flows into the Gulf of Finland in Loksa.
The lake is famous for the fluctuations of the water-level. Due to karst fissures in the bottom of the lake it has remained completely dry in some years, revealing the bottom and destroyed the fish. Due to fluctuating water-level floating islands have been formed in the Southern part of the lake, that change their location. Among numerous springs the most famous ones are called Kaieallikas and Külmaallikas, which are the ground for several legends and lore.