Water flows are consensually scraped from The Norwegian Water Resources and Energy Directorate and Friends of the Lysaker catchment. Weather forecasts are provided by The Norwegian Meteorological Institute.
Data on river sections, optimal water levels, and similar information is mostly based on Tore Nossum's excellent guides. If you haven't checked out his guides yet, be sure to do so - they're free! The whitewater kayaking community is deeply indebted to him for his efforts to create these guides.
What is nokken?
Nokk are male water spirits who play enchanted songs on the violin, luring women and children to drown in lakes or streams. Not all of these spirits were necessarily malevolent; in fact, many stories exist that indicate at the very least that nokker were entirely harmless to their audience and attracted not only women and children, but men as well with their sweet songs. Nokken sings about his loneliness and his longing for salvation, which he purportedly never shall receive, as he is not "a child of God". In a poem by Swedish poet E. J. Stagnelius, a little boy pities the fate of the nokk, and so saves his own life. If you brought the nokk a treat of three drops of blood, a black animal, some vodka or snus (wet snuff) dropped into the water, he would teach you his enchanting form of music.
It is difficult to describe the actual appearance of the nokk, as one of his central attributes was thought to be shapeshifting. Perhaps he did not have any true shape. He could show himself as a man playing the violin in brooks and waterfalls but also could appear to be treasure or various floating objects. Nokk are said to grow despondent if they do not have free, regular contact with a water source. The nokk was also an omen for drowning accidents. He would scream at a particular spot in a lake or river, in a way reminiscent of the loon, and on that spot a fatality would later take place(Loosely paraphrased from Wikipedia.)