Rivers

Along the ridge of Rila Mountain passes the main watershed between the catchments of the Danube (Black Sea catchment) and the Aegean catchment. The main watershed successively divides the catchment basins of the Iskar River and the Struma River, the Iskar River and the Mesta River and the Iskar River and the Maritsa River. From West to East the watershed passes successively through Malyovitsa Peak - 2,729 m, Shishkovitsa Peak - 2,669 m, Musala Peak - 2,925 m and east of Borovets Resort it extends beyond the park. The length of the watershed is about 76 km. The main watershed divides the National Park into two parts - North, from which the waters flow through the Iskar River to the Danube River and from there to the Black Sea (Black Sea catchment area) and South, from which the waters flow to the White Sea (White Sea catchment area). . The main river, which originates from the territory of the National Park and belongs to the Black Sea catchment area, is the Iskar River. The main rivers that belong to the Aegean catchment area are the Mesta River and the Maritsa River. A significant part of the park forms an outflow to the Struma River, which flows into the White Sea.
The waters in Rila National Park are extremely soft with one of the lowest electrical conductivities in Bulgaria - varying so far from 6.1 µS / sm (in one of the measurements of Karagyol Dam - in phytobenthos sampling) to 79 µS / sm (Leevshtitsa River). ). Practically in the Rila lakes and dams no electrical conductivity of more than 20 µS / sm has been measured anywhere.
There are no established problems with dissolved oxygen (resp. Oxygen saturation), which is about 9 mg / l and resp. about 100% saturation, which is normal for this type of water;
The acidity of the water (measured as pH) varied between 7 and 8, and only in Karagyol Dam and Yonchevo Lake was the pH measured just below 7 (pH = 6.81), which is normal for this type of water.
All inorganic forms of nutrients (ammonium nitrogen, nitrite nitrogen, nitrate nitrogen, orthophosphates) are below the limit of detection of analytical methods and show that the river and lake waters in Rila National Park are extremely poor in nutrients (oligotrophic and ultraoligotrophic).
Total phosphorus (organically bound and inorganic) is also below the limit of quantification of the analytical method in river and lake waters within the boundaries of Rila National Park.
Only low values ​​of total nitrogen (organically bound and inorganic) were registered at some of the points: for rivers ranging from <0.2 mg / l to 0.93 mg / l (Kriva River); in lakes ranging from <0.2 mg / l to 1.9 mg / l (Karagyol dam). These final values ​​exceed the norms for good condition laid down in Ordinance H-4, but it should be borne in mind that these norms are for average annual values ​​and it is not correct to compare them with one-time measurements, especially in the period close to snowmelt. (when is the biogenic annual maximum of dissolved forms).
The values ​​of COD (chemical oxygen demand), which is an indicator of total water pollution, are in most cases below the limit of quantification of the method of 5 mg / l or slightly above it, reaching a maximum of 7.2 mg / l (Leevshtitsa River) , which are extremely low values ​​for this indicator.
The indicator for total organic pollution (BOD5 - biochemical oxygen demand) has very low values, ranging from <0.5 mg / l (Beli Iskar dam) to 1.59 mg / l (Karagyol dam), which shows waters with very high drinking water. quality.
The river and lake waters in Rila National Park are characterized by very high quality in the main groups of physicochemical parameters (oxygen status, acidification / acidification, nutrient load, salt regime), and no cases of clear anthropogenic pollution have been registered in the park. (in the surveyed 17 rivers and 18 lakes).