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Surface water and groundwater resources
There are more than 12 400 rivers in the country (GWP, 2011). The major rivers are Daugava, Lielupe, Gauja, Venta, and Salaca.
According to the Law on Water Management Latvia is divided into four river basin districts (RBD) (EC, 2015; EEA, 2015):
- Daugava, including two basins:
- Daugava river basin: Its total area is 87 900 km², of which just over one quarter is located in Latvia. The Daugava river rises in the Russian Federation, flows through Belarus (where it is called the Western Dvina), enters Latvia in the southeast and flows northwest to the Gulf of Riga. Several tributaries enter the Daugava river in Latvia, including three large ones: Ogre, Aiviekste and Dubna. The Rezekne and Pededze rivers are two important tributaries of the Aiviekste river.
- Velikaya river basin: This basin consists of a number of smaller rivers flowing into the Velikaya in the Russian Federation. The Velikaya river is a major tributary of Lake Peipus and belongs to the basin of the Narva river which is predominantly in Estonia. Its area within Latvia is 3 200 km². The main tributaries flowing from Latvia to the Velikaya river are the Sinyaya (Zilupe in Latvian) river which originates in Belarus, the Ultroya river and the the Kukhva (Kukova in Latvian) river. The total discharge of the Velikaya amounts to 4 200 million m³/year, of which 16 percent is generated within Latvia.
- Gauja, including two basins:
- Gauja river basin: Its total area is 8 900 km², of which almost 95 percent is situated in Latvia. The Gauja river rises in the Vidzeme upland and flows east, then turns northwest, becomes the border between Latvia and Estonia for a short distance, and then flows southwest to the Gulf of Riga.
- Salaca river basin: It covers the north of the country, near the border with Estonia. Its total area is 3 600 km², of which more than 95 percent is located in Latvia. The Salaca river rises in Lake Burtnieks in the north and flows west to the Gulf of Riga.
- Lielupe, including one basin:
- Lielupe river basin: Its total area is 17 600 km², of which about half is situated in Latvia. The Lielupe river rises in Lithuania, enters Latvia in the south and flows north to the Gulf of Riga through the most fertile regions of the country. It has many tributaries, the most important being the Memele, Lecava and Svete. The Musa river is a tributary of the Memele river.
- Venta, including three basins:
- Venta river basin; Its total area is 11 800 km², of which almost two thirds is situated in Latvia. The Venta river rises in Lithuania, enters Latvia in the southwest and flows north through the Kurzeme lowland to the Baltic Sea. The Venta has many tributaries, but only one of them, the Abava river, exceeds 100 km in length.
- Coastal west basin: It includes the small river basins at the west coast with a total area of 5 100 km². It includes rivers such as the Barta, Durba, Riva and Uzava, which flow to the Baltic Sea.
- Coastal north basin: It includes the small river basins within the coastal lowland on the opposite shores of the Gulf of Riga. Their combined area is 3 800 km². This area includes rivers such as the Irbe, Stonde, Roja, Svetupe and Vitupe.
Internal renewable surface water resources (IRSWR) are estimated at 16 540 million m³/year (Table 2) and incoming surface water resources at 17 999 million m³/year. Therefore, the total renewable surface water resources (RSWR) are estimated at 34 539 million m³/year.
Depending on the physical and geographical conditions, a large part of the river discharge comes from either snow melt, groundwater or direct surface runoff. About 50-55 percent of the waters of the Daugava, Venta, Lielupe and Musa rivers is melted snow, while for the Gauja and Amata rivers it is 35-40 percent. About 10-20 percent of the flow of some tributaries of the Lielupe (Memele and Svete) and the Aiviekste tributary of the Daugava is fed by groundwater, while for the Daugava and Gauja rivers it is 35-40 percent. In the Kurzeme peninsula and in the middle uplands, direct surface runoff accounts for 40 percent of flow of the rivers, while in the Zemgale plain it represents 20-30 percent.
Internal renewable groundwater resources are estimated at 4 700 million m³/year. Part of the groundwater flows to the sea or is withdrawn by wells, and part is drained by the surface network. That part of the groundwater flow which does not contribute to the total IRWR (overlap) is estimated at 4 300 million m³/year. This brings the internal renewable water resources (IRWR) to 16 940 million m³/year (16 540+4 700-4 300) and the total renewable water resources (TRWR) to 34 939 million m³/year (34 539+4 700-4 300) (Table 3).
Artificial groundwater recharge takes place in several well fields in Baltezers that are used for Riga city water supply. A hydrogeological modelling and field investigation has shown that artificial recharging of groundwater resources in Baltezers does not put at risk groundwater quality to exceed permitted parameters’ values for drinking water. (LEGMA; 2005; Spalvinš et al, 2008).
In 2009, produced municipal wastewater was estimated at 282 million m³, treated municipal wastewater at 128 million m³ and direct use of treated municipal wastewater at 12 million m³.
Lakes and dams
There are about 2 250 lakes with a total area of about 850 km², most of them being small lakes with more than 10 000 lakes having a surface area of less than 1 ha (LEGMA; 2005). About 36 percent of the lakes in Latvia are located in the Latgale upland in the southeast of the country. Lubans lake in the Daugava river basin is the largest lake with a surface area of almost 100 km².
Dams have been constructed for two main reasons: to control floods and to build hydro-electric power stations. Before the Second World War, about 300 such stations had been built. After the Second World War, the construction of another 547 small stations was planned, but only 267 were built. At present, no small power stations are functioning, though the reservoirs still exist.
Three large hydropower dams have been constructed on the Daugava river, with a total full reservoir capacity of 1 005 million m³: (1) the Kegums reservoir, with an area of 25 km² and a full capacity of 157 million m³, was constructed in 1947; (2) the Plavinas reservoir, with an area of 35 km² and a full capacity of 509 million m³, was constructed in 1965; (3) the Riga reservoir, with an area of 42 km² and a full capacity of 339 million m³, was constructed in 1976.