History of the delta In the
Paleozoic, 300–400 million years ago, all the delta region was covered by a sea. Modern relief, eminences, were formed by glacial scouring. Its retreat formed the
Littorina Sea, the level of which was higher than its successor the Baltic Sea. Then, the
Tosna was flowing in the modern lower half of the Neva as today, into the Litorinal Sea. In the north of the
Karelian Isthmus, the sea was united by a wide strait with
Lake Ladoga. The
Mga then flowed to the east, into Lake Ladoga, near the modern source of the Neva. Thus the Mga then was separate from the Tosna/lower-Neva basin. Near the modern Lake Ladoga, by
glacial rebound land rose faster, and an endorheic lake briefly formed. This overspilled, eventually the whole Mga valley and thus broke into the western valley (the valley of the Tosna/lower-Neva). The Ivanovo
rapids of the modern Neva were created at the breakthrough. According to early books, the breakthrough may have been about 2000 BC, but according to more recent research, this happened at 1410–1250 BC, making the river rather young. The delta was formed at that time, technically a pseudodelta, as not from accumulation of river material but by scouring past sediments.
Topography and hydrography The Neva flows out of Lake Ladoga near
Shlisselburg, flows through Neva's lowlands and discharges into the
Baltic Sea in the
Gulf of Finland. It has a length of , and the shortest distance from the source to the mouth is . The river banks are low and steep, on average about and at the mouth. There are three sharp turns: the Ivanovskye rapids, at Nevsky Forest Park of the Ust-Slavyanka region (the so-called "Crooked Knee"), and near the
Smolny Institute, below the mouth of the river
Okhta. In the Neva basin, rainfall greatly exceeds evaporation; the latter accounts for only 37.7 percent of the water consumption from the Neva and the remaining 62.3 percent is water runoff. Since 1859, the largest volume of was observed in 1924 and the lowest in 1900 at . Due to the uniform water flow from Lake Ladoga to the Neva over the whole year, there are almost no floods and corresponding water rise in the spring. The Neva freezes throughout from early December to early April. The ice thickness is within Saint Petersburg and in other areas. Ice congestion may form in winter in the upper reaches of the river, this sometimes causes upstream floods. Of the total ice volume of Lake Ladoga, , less than 5 percent enters the Neva. including the pools of Lake Ladoga and Onega (281,000 km2). The basin contains 26,300 lakes and has a complex hydrological network of more than 48,300 rivers, however only 26 flow directly into Neva. The main tributaries are
Mga,
Tosna,
Izhora,
Slavyanka and Murzinka on the left, and
Okhta and
Chyornaya Rechka on the right side of Neva. Several centuries ago, the whole territory of the Neva lowland was covered by
pine and
spruce mossy forests. They were gradually reduced by the fires and cutting for technical needs. Extensive damage was caused during
World War II: in Saint Petersburg, the forests were reduced completely, and in the upper reaches down to 40 to 50 percent. Forest were replanted after the war with spruce, pine,
cedar,
Siberian larch,
oak,
Norway maple,
elm, America,
ash,
apple tree,
mountain ash and other species. The shrubs include
barberry,
lilac,
jasmine,
hazel,
honeysuckle,
hawthorn,
rose hip,
viburnum,
juniper,
elder,
shadbush and many others. Nowadays, the upper regions of the river are dominated by
birch and pine-birch grass-shrub forests and in the middle regions there are swampy pine forests. Because of the rapid flow, cold water and lack of quiet pools and aquatic vegetation the diversity of fish species in Neva is small. Permanent residents include such undemanding to environment species as
perch,
ruffe and
roaches. Many fish species are transitory, of which commercial value have
smelt,
vendace and partly
salmon.
Floods Floods in St. Petersburg are usually caused by the overflow of the delta of Neva and by surging water in the eastern part of
Neva Bay. They are registered when the water rises above with respect to a gauge at the
Mining Institute. More than 300 floods occurred after the city was founded in 1703. Three of them were catastrophic: on 7 November 1824, when water rose to ; on 23 September 1924 when it reached , and 10 September 1777 when it rose to . Besides flooding as a result of tidal waves, in 1903, 1921 and 1956 floods were caused by the melting of snow.
Ecological condition The Federal Service for
Hydrometeorology and Environmental Monitoring of Russia classifies the Neva as a "heavily polluted" river. The main pollutants include
copper,
zinc,
manganese,
nitrites and
nitrogen. The dirtiest tributaries of the Neva are the Mga, Slavyanka, Ohta, and Chernaya. Hundreds of factories pour wastewater into the Neva within St. Petersburg, and
petroleum is regularly transported along the river. The annual influx of pollutants is 80,000 tonnes, In 2008, the Federal Service of St. Petersburg announced that no beach of the Neva was fit for swimming. Cleaning of wastewater in Saint Petersburg started in 1979; by 1997, about 74% was purified. This rose to 85% in 2005, to 91.7% by 2008, and Feliks Karamzinov expected it to reach almost 100% by 2011 with the completion of the expansion of the main sewerage plant. == History ==