584 THE FRESH- WATER LOCHS OF SCOTLAND 
topography of the limestone region south of Lake Onega. These 
lakes are connected by natural channels, but as the basins are not 
filled and emptied simultaneously, the direction of flow in the 
channels changes from time to time. The largest of the lakes, 
Shimozero, discharges its waters into an underground abyss some 14 
miles to the east, and is sometimes completely empty by November. 
The Dolgozero, at the other end of the system, is drained in a similar 
manner. These lakes and others in the vicinity, differ from the Lake 
of Zirknitz ^ (the classical example of an intermittent lake) in not 
being filled again by the same channel by which they are emptied, 
but by ordinary above-ground agencies, the process sometimes taking 
as long as seven years. 
Lake Ilmen, which also belongs to the Neva system, is really 
nothing more than a permanent inundation formed by a large number 
of rivers which meet at a point whence the outlet is not large enough 
to carry off the whole of the water. The lake lies at an altitude of 
107 feet, and has an area of 358 square miles, being 30 miles in length 
from east to west by 24 miles in maximum width, but the depth does 
not exceed 30 feet. Its waters are generally muddy, as are also those 
of the River Volkov, its outflow, which is the chief affluent of Lake 
Ladoga. The principal streams meeting in Lake Ilmen are the 
Shelon, the Lovat, and the Msta, which brings it into communication 
with the Volga. 
River Narova. Lake Peipus (or Chudskoye) is a triple lake, the northern part 
of which is Lake Peipus proper, the southern I^ake Pskov, and the 
connecting channel, 40 miles long by 3 to 10 miles wide, Lake 
Teploye. The area of the whole lake to the end of Lake Pskov is 
1356 square miles,^ and the area of the northern portion (Lake Peipus 
proper) is 1082 square miles. The maximum depth, which was 
found in Lake Teploye, is 90 feet, and the altitude is 97 feet above 
sea-level. It receives the Rivers Velikaya and Embakh, and its 
outlet is by the Narova to the Gulf of Finland. The area of Lake 
Peipus has been considerably increased in consequence of the drainage 
of the surrounding country having been conveyed into it more abund- 
antly, through the construction of 1200 miles of artificial cuttings. 
1 Lake Zirknitz is situated in tlie Karst region in Austria. For a number of 
seasons together the bed of the lake may remain quite dry and be used for cultiva- 
tion, while at other times it is occupied by waters teeming with fish. The under- 
ground outlets for the superficial water are sometimes comparatively empty, 
sometimes overflowing. In the former case the fissures communicating with these 
periodical lakes serve as channels to lead away the water ; in the latter they serve 
as vents to pour the water on the plain. 
- Helmersen, cited by K. Peucker, " Europaische Seen nach Meereshohe, Grosse 
und Tiefe,'^ Geogr. Zeitschr., Bd. ii. j). 612, Leipzig, 1896. 
