Complex sentence: This lake falls completely dry in autumn and remains empty until being filled by a small river of just about half a mile in length, falling some 100 meters in level, that will start flowing around May depending on temperatures and snow melt rate in the mountains.
Split into simpler sentences: This lake falls completely dry in autumn and remains empty until being filled by a small river of just about half a mile in length, covering some 100 meters difference in altitude. That river will start flowing around May depending on temperatures and snow melt rate in the mountains.