The land-fast sea-ice cover on the surface of Arctic estuaries in winter leads to a fundamental dynamical change in their hydrodynamics. Instead of tide-independent wind forcing, an oscillating tidal stress acts at the surface, with a magnitude that is comparable to the usually dominating bed stress. For ice-free estuaries, there exist wellestablished theories to explain estuarine circulation and mixing, two strongly interlinked processes that determine the structure and position of the salt wedge and dominate sediment transport patterns and biogeochemical transformations. For ice-covered estuaries such theories are missing, despite the fact that some of the world's largest estuaries are situated in the Arctic. With this, a major driver for hydrodynamics, sediment transport and biogeochemistry of Arctic estuaries as well as feedbacks to the dynamics of the ice-cover itself are not well understood. The present project intends to shed light into this yet unexplored research field by means theoretical considerations as well as idealised and realistic numerical modelling. The focus estuary will be the Gulf of Ob, the world's longest estuary, which is discharging into the Kara Sea of the Arctic Ocean. Under the perspective of a rapidly warming Arctic with an expectedly reduced estuarine ice cover, this new theory of Arctic estuaries is more than timely.