Lake Jági, Pilisszentiván

Jagi Lake, also known as Vadász-réti reservoir, is an artificial lake in the Buda Hills, within the Danube–Ipoly National Park, in the administrative area of Pilisszentiván. The lake was created by enlarging the Vadász-réti stream, a tributary of the Aranyhegyi stream, originally for fish breeding and fishing purposes, but it is now a nature protection area.

The lake permanently dried up in the summer of 2021, although several attempts were made to promote natural refilling; however, the lack of precipitation prevented this.

Name

Jagi Lake received its name from the local Swabian (German) name of the nearby Vadász-rét and the Vadász-réti stream (Jägerwiesen), and although the name "Vadász-réti reservoir" is technically more official, the name Jagi Lake (or simply Jagi) is more commonly used among locals, hikers, and nature enthusiasts.

Origin

In the Vadász-réti stream, which runs under the Nagy-Szénás block, two valley dams were first built in the 1940s with the aim of breeding trout in the resulting small lakes. The plan was soon undermined by large-scale mining developments that began in 1948 in the area, which significantly reduced the water flow of the Vadász-réti stream, and several springs in the region even completely dried up. Of the two lakes formed behind the dams, the upper one completely dried out, and the lower one also began to undergo significant eutrophication.

In the 1980s another development attempt was made in the area, this time involving a Budapest fishing club, after the cessation of earlier mining activities in the region had slightly increased the stream's water flow. At that time two smaller dams were also built behind the main dam that closes the valley, and a sluice was installed in the main dam to ensure the possibility of fishing. However, this plan also was not long‑lived, presumably because the area was declared protected, which happened around 1990.

Presumably because the main dam that closes the valley was not properly constructed, the water repeatedly damaged the area around the main dam's sluice and found its way out at other points of the dam, so in rainy years the lake's water continuously drained, while in drier periods the basin even completely dried up. Because of this the Pilisszentiván municipality developed several plans for the lake's restoration, and finally the solution was adopted to remove the dam's sluice and fill the removed structure with clean clay, as well as to reinforce the water‑side faces of the main dam and the second dam with a 1 metre and a 50–60 cm thick clay layer, respectively.

The water retention action completed in 2006 essentially stabilized the amount of water in the lake, but the year 2010 showed that the region’s water flow can still cause surprises during rainy periods. In June 2010, because of heavy rain, the swollen Vadász-réti stream again swelled into a lake behind the second dam, which by then had become almost completely overgrown, then it broke through it, so the outflow damaged the main dam as well, and then rushed down a deep trench dug along the roughly one‑kilometre‑long forest road leading to the lake, rendering the road section impassable for several weeks. The main dam was restored later that year, but this event indicates that a similar damage case cannot be completely ruled out in the future.

Flora and Fauna

The fauna of Lake Jági is characterized primarily by a rich amphibian and aquatic insect fauna. Among the native frog species, the brown frog, the common toad, the green tree frog, the forest frog and the red‑spotted newt occur here, and the spotted newt and the marsh turtle also live here. Among the more interesting aquatic insect species are the back‑swimming water boatman, the common water scorpion, the marginal water beetle, the water strider, the mayfly and other species. Among the molluscs, the area is typical for the horse mussel, the striped mussel, as well as the protected Hungarian mud snail.

The bird life observable around the lake is also species‑rich, partly thanks to the strict protection of the large‑peat region, partly because neighboring forest patches have hosted several nest‑box colonies for years. In the wet habitats of the area the most characteristic species are the reed warbler, the water rail and the kingfisher, and the grey heron also occurs here not rarely.

The protected fern species *Pteridium aquilinum* var. *pseudoplatanus* was also found in the area, which represents the most eastern known occurrence of this species in Hungary.

The Jági Trail

In cooperation with several local and regional civil organisations, the Directorate of the Danube–Ipoly National Park established a trail in the early 2000s in the Lake Jági area, with six stations, the second of which was set up directly beside the lake. In addition to informational boards, the station includes a resting place, a fire‑pit and a bird feeder, and on the eastern shore of the lake a wooden shelter was also built. The resting place is regularly used by staff of the Danube–Ipoly National Park for bird‑ringing demonstrations as well as for research‑purpose bird ringing, and national park events are regularly held here on World Water Day.