Lake Baikal receives over 300 streams, mostly short mountain torrents, besides the Upper Angara, which enters its north-east extremity, the Barguzin, on the east, and the Selenga on the south-east. Its only outflow is the lower Angara, which issues through a rocky cleft on the west shore. The Irkut no longer reaches the Baikal, though it once did so. After approaching its south-west extremity it abandons the broad valley which leads to the lake, and makes its way northwards through a narrow gap in the mountains and joins the Angara at Irkutsk.
With the exception of the delta of the Selenga, Lake Baikal is surrounded by lofty mountains. The Khamar-daban border-ridge (the summit of a mountain of the same name is 5300 ft. above the lake), falling with steep cliffs towards the lake, fringes it on the south; a massive, deeply-ravined highland occupies the space between the Irkut and the Angara; the Onot and Baikal ridges (also Primorskiy) run along its north-west shore, striking it diagonally; an Alpine complex of yet unexplored mountains rises on its north-east shore; the Barguzin range impinges upon it obliquely in the east; and the Ulanburgasu mountains intrude into the delta of the Selenga.
It is certain that in previous geological ages Lake Baikal had a much greater extension. It stretched westwards into the valley of the Irkut, and up the lower valleys of the Upper Angara and the Barguzin. Volcanic activity took place around its shores at the end of the Tertiary or during the Quaternary Age, and great streams of lava cover the Sayan and Khamar-daban mountains, as well as the valley of Irkut. Earthquakes are still frequent along its shores.
The fauna, explored by Dybowski and Godlewski, and in 1900-2 by Korotnev, is much richer than it was supposed to be, and has quite an original character; but hypotheses as to a direct communication having existed between Lake Baikal and the Arctic Ocean during the Post-Tertiary or Tertiary ages are not proved. Still, Lake Baikal has a seal (Phoca vitulina, Phoca baikalensis of Dybowski) quite akin to the seals of Spitsbergen, marine sponges, polychaetes, a marine mollusc (ancilodoris), and some marine gammarids. The waters of the lake swarm with fish (sturgeons and salmonidae), and its herring (Salmo omul) is the chief product of the fisheries, though notably fewer have been taken within the last forty or fifty years. Plankton is very abundant. The little Lake Frolikha, situated close to the northern extremity of Lake Baikal and communicating with it by means of a river of the same name, contains a peculiar species of trout, Salmo erythreas, which is not known elsewhere. Generally, while there is a relative poverty of zoological groups, there is a great wealth of species within the group.
Of gammarids, there are as many as 300 species, and those living at great depths (330 to 380 fathoms) tend to assume abyssal characters similar to those displayed by the deep-sea fauna of the ocean.