The Paz River (Patsojoki in Finnish, Pasvikelva in Norwegian) is the major waterway in Pasvik Inari region and Pasvik Reserve.  It is noteworthy that national borders of the three countries meet there.  The river flows out of Inari Lake in Finland; the Upper Paz is in Russia and it flows into Varanger Fjord, Norway.  The Russian-Norwegian border is in the midstream of the river.       

The Paz is a typical lake and river system of Fennoskandia.  It has plenty of bays and watercourse expansions; their names are Heuhenjarvi, Vouvatusjarvi, Bossojarvi and Salmijarvi. The watercourse expansions are connected by a great number of rapids and shallows.  The river has great power potential; the water level difference across it is 199 meters.  A cascade of HEP stations was built on the big rapids in the period from 1950 till 1978.   Five power stations were built in Russia (Kaitakoski, Janiskoski, Rajakoski and Borisoglebsk) and two in Norway (Skogfoss and Melkefoss).  As a result lakes were replaced by water basins. The riverbed remained intact only in some places (Heunhenjarvi and Fjervann) within the confines of the nature reserve.   The area is the best for water fowls in the Paz River valley.   The south part of Russian Pasvik Reserve was put on the Perspective list of Ramsar Convention due to the abundance of birds (about 130 species).  Norwegian Pasvik Nature Reserve was awarded a Ramsar Diploma.             

The river valley is 147 kilometers long. A part of the reserve is in the centre of the valley and it stretches for 44 kilometers.  Several big tributaries (Nautsijoki, Sejgijoki, Kornetijoki and Laukkujoki) and plenty of streams flow into the river.  There are about fifty streams within the confines of the reserve.  The Menikkajoki River is in the north of the reserve.   It is an ex-branch of the river which was regulated as a result of non-overflow dam construction or Skogfoss HEP station.  The regime of the Menikkajoki has greatly changed: the river is shallow in the upstream.   

The water catchment area of the Paz River is about 20890 km2.  Nearly 70 per cent of the water catchment area is in Finland, 25 per cent is in Russian and 5 per cent is in Norway.  The reserve's water area is 3020 hectares or 20,5 per cent of the total reserve's area.  The river ranges in width from 200 meters in Jordanfoss to 4 kilometers in Vaggatem; it is 1,5 meters deep in Heuhenjarvi Lake and it can be up to 21 meters deep in water storage reservoirs.    

Annual variation of the Paz River water level is defined by floods in spring, a higher flow in autumn and mean water level in winter.  Snow, rain and interflow are main feeding sources of the river.  It starts freezing over at the end of October or at the beginning of November and it is icebound for about 200 days.   Ice drift starts in the middle or at the end of May.  Ice varies in thickness from one cm in the vicinity of shallow places to one meter close to the shores.  Big rapids and diversion canals of the HEP stations are ice free in winter.    

There are 25 lakes in the reserve; they are mainly of moraine origin.  The majority of them are shallow; their banks are rocky, sandy and silty or peaty.  Kaskamajarvi Lake is the biggest; its area is 250 hectares; it is of glacial and tectonic origin.  It appeared in the tectonic fracture of the Earth's crust and was affected by the glacier later.  That is why it is rather deep (more than 20 meters) and has the rocky bottom and shores.   The total area of the reserve's lakes is 439 hectares.    

Swamps occupy 3741 hectares or 25 per cent of the reserve' area.  Together with water basins they form different wetland sites; the total area of the latter is 6761 hectares.