What's that really interesting smell at water leve

Walking along the stream today Rusty was always smelling at the edge of the water. I thought at first he was thirsty but no, even with his mouth virtually in the water it was sniffing  that was important.
I suppose there are always interesting smells around streams with animals living in/on the banks and other animals coming down to drink but in this stream the beaver are definitely a smell source.
Here you can see a dam, creating a pool protecting the lodge which is just visible, slightly right of centre at the top of the picture.
It must be a very different world when smell is such an important sensory input.