Hydrology is traditionally considered to be the study of properties, processes and distribution of freshwater at or near the Earth’s surface.  As such, hydrologists tend to restrict their focus to distilled water after it evaporates into the atmosphere and before it bleeds off into the ocean.  Here, I will discuss what happens to that fresh water within the ocean.  In keeping with the theme of this symposium, I’ll focus attention on northern high latitude seas.  Insofar as climate change and variability in the ocean are concerned, freshwater runoff provides two important properties – buoyancy and coloured dissolved organic matter (CDOM) – that provide enormous scope for feedback. The vertical structure of the Arctic Ocean is controlled partly by global hydrological processes like the return of fresh water from the Pacific to the Atlantic, and partly by smaller scale processes like the storage of runoff over the arctic shelves and basins.  These processes operate on the supply of buoyancy to convecting regions of the North Atlantic. Along its pathway, runoff interacts with the sea-ice distillation engine leading to negative or positive feedbacks in buoyancy production.  On the other hand, terrestrial CDOM, as it passes through the ocean, controls the penetration of light.  Change in CDOM inventory and distribution in northern oceans offers far greater potential to alter UV exposure than does increased incident UV due to destruction of stratospheric ozone.  The presentation will be illustrated with data collected during the past two decades from the Beaufort Sea and Hudson Bay.