Finnish lakes, rivers, the Baltic and wetlands–our aquatic ecosystems–are surveyed, assessed and studied using natural sciences and ecological data. We argue that this approach is leaving out a key repository of information–the traditional knowledge of waters. By reviewing aspects, qualities and depth of knowledge across Finland from mid-boreal ecosystems in Eastern Finland to large Saimaa lake system to Western Finnish lakes we demonstrate a potentially reformative approach to aquatic assessments. Cultural indicators, baseline data from oral histories, linguistic diversity and behaviour of species on lakes based on traditional knowledge observations have the potential to greatly enhance our understanding of ecosystem health. Of special importance are the spawning locations of fish known through traditional knowledge, but often ignored by science-based assessments.