Simplify the following text for a Grade 5 native English-speaking person.
Original: Rising at the height of 1640 m at its sourcenick Baikal Mountains south of the Central Siberian Plateau , 7 k west of Lake Baikal , the Lena flows northeast , being joined by the Kirenga River , Vitim River and Olyokma River .
Simplified: The Lena begins in the Baikal Mountains , a mountain range in central Siberia , at a point 7 m west of Lake Baikal . The Lena flows northeast .