Complex sentence: Before the dam was built, the Yenisey in that area was free from ice around 196 days per year; now it remains free from ice the whole year round up to 300 to 400 km downstream.
Split into simpler sentences: Before the dam was built, the Yenisey in that area was free from ice around 196 days per year. Now it is free from ice the entire year up to 300 to 400 km downstream.