I think you nailed the start of this piece, it does really evoke a peaceful rivers, and things do pick up with the percussion and war drums. I don't think it gets quite to the mood of a ominous rolling storm like you described, but that may just be me. I think it may come through better if there was a break in the middle of the piece for another section or a key change to really accentuate the darker nature of a storm. That being said, I think the melodies and choice of instruments really bring out the "spirit of the north" so you definitely got the theme right. At some points the layering with the instruments felt a little clashing and that may be due to them occupying too similar of volumes or frequencies. I think this might be easily resolved with some stereo panning but I'm sure there's other ways too. Overall great work! I found the piece very listenable!