The Dragon of Lake Suwa appears only in winter, not every year, and increasingly less due to global warming. Simply, it is a natural phenomenon due to crushes of big ice plates in the lake, which makes huge noises and appearances of long streaks on the surface.
Old times, people found meanings (logos) in it. In the dark winter night, loud squeaking and crunching noises from the big lake were echoing the surrounding mountains. The sound reached not only the lakeside villages but many miles away! In the dark, it must have been scary and inspiring with awe for those who heard it.
People thought that gods were walking across the lake in the shape of Dragon because the shape of the streak looked like a white dragon! It was natural to think there was a powerful existence behind the phenomenon.
This newspaper article was from 2018 when the Dragon appeared after five years of absence! A few years before that (December 2015), my mother and I had a weekend trip to Suwa Taisha (Grand Shrine). We did not get to see the ice dragon but we met this Shinto priest (in the pic) by chance at his home shrine Tenaga Shrine. He was reading or practising Norito (Shinto ritual prayer) all alone in nature (the shrine was beside woods).
There was only my mom, me and the priest who was reading Norito outside in the woods. It was a magical moment.
An article of the Asahi Shimbun on 24 February 2018