How long exactly is the Enisei River?  Well, that depends!  There are two major sources of the river:  the Bolshoi (large) Enisei (aka Bii-Khem or Бии-Хем) which originates in the Tuva Federal Republic on the border with Mongolia; the Malyi (small) Enisei (aka Kaa-Khem orКаа-Хем) which originates in the rift valley of Mongolia.  The two branches meet at Kyzyl, the capitol of Tuva.  From that point, the river is later joined by the Khemchik River, the Abakan River and then eventually at Strelka by the Angara River which drains Lake Baikal.  So, to make long matters short, if the Bolshoi Enisei River is considered the main source of the Enisei, then the river is a little over 2,500 miles (~4,000 km) long.  Thought that I'd throw this in for good measure:  The longest continuous "stream following the Yenisei-Angara-Selenga-Ider River course is about 5,500 km."