Complex sentence: In the early years of operation, the Mayak plant released vast quantities of radioactively contaminated water into several small lakes near the plant, and into the Techa River, whose waters also ultimately flow into the Ob River.
Split into simpler sentences: In the early years of operation, the Mayak plant released vast quantities of radioactively contaminated water into several small lakes near the plant. These lakes draines in the Techa River, whose waters also ultimately flow into the Ob River.