Our cameras continue to follow the wildlife dramas in Yellowstone, and Spring brings many new challenges. While the brutal cold temperatures and deep snow of winter have gone, the weather is still erratic and there's the impending danger of the Thaw. When temperatures are high enough to melt the snowpack, millions of tonnes of water will cascade down the mountain, bulldozing everything in its path. Wildlife cameraman Jeff Hogan discovers a nest of Great Gray owls. There are three chicks, but one is a runt, significantly smaller than its siblings and at risk of starvation. And when the worst storms for years hit the Beartooth Mountains, Jeff wonders whether it can hang on. Down on the Snake River the family of beavers is busy making the most of the Spring vegetation. Infra red cameras give Jeff and host Kirk Johnson a secret view of life inside the lodge, as the beavers squabble over food. Kirk also dons a dry suit to head underwater to admire the engineering skills of these rodents. Massive boulders and branches are there to strengthen their dams, but will they be enough to withstand the Thaw. When the torrent of water begins to hit, Kirk joins a hydrologist to help measure the water flow in the rivers and work out how this year's weather has affected the Thaw. And bear expert Casey Anderson follows a mother grizzly and her cubs - and explains that the biggest dangers come not from other predators, but surprisingly from their own kind.