S1, E2: As winter turns to spring, temperatures rise and Yellowstone bursts into life. Beavers feast on new green shoots, baby bison take their first faltering steps, and grizzly bear cubs emerge from dens to explore this new green world. But spring is also a perilous time. Wolves are hungrier than usual after slim winter pickings, and rising temperatures melt the vast mountain snow pack, sending a million tons of water flooding into the rivers. Only the toughest will survive.