Estuarine tidal mixing affects the size and depth of large river plumes in the Arctic Ocean. The Yenisei and Khatanga rivers have similar plume sizes despite different discharge rates due to tidal forces. The Khatanga River forms a diluted, deep plume with low freshwater content, covering a large area. In contrast, the Yenisei River creates a shallow, strongly-stratified plume with high freshwater content and a smaller area. Tidal mixing in estuaries plays a crucial role in determining the freshwater fraction of river plumes, influencing their spread in coastal seas. This factor is as important as river discharge rate and wind conditions in shaping the size and depth of river plumes.