The Ythan Estuary (Scottish Gaelic: Inbhir Eithein) is the tidal component of the Ythan River, emptying into the North Sea north of Aberdeen, Scotland. The estuary’s tidal action extends a inland and has characteristic widths of between and . Besides the tidal channel there are interfaces to the upland dunes including mudflats, sand beaches and shingle flats. Reaches of salt marsh occur, but they are primarily near the Waterside Bridge (crossing of the A975 road) and the mouth of the Tarty Burn, a small tributary river. Based upon the habitat of the moorland bordering the east of the Ythan River near the mouth, this estuary is the most significant coastal moorland in the northern United Kingdom. The Ythan Estuary is a designated Site of Special Scientific Interest (SSSI) and appears as site no. 939 on the Ramsar list of wetlands of international importance with Meikle Loch. There are 50 breeding pairs of common shelducks in the estuary, and there is a mixed tern breeding colony on the east shore from the mouth comprising Arctic, Sandwich, little and common terns. Leisure activities including birdwatching, fishing, canoeing and wildfowling are permitted. There are hides to watch waders (best at low tide), and a wildfowling code of conduct with a voluntary refuge area south of the main bridge over the estuary (A975). Visitors can fish for estuarine species such as flounder for free but are advised not to disturb birds during the breeding season. Game fisher's catching salmon and sea trout is by permit only. Topography and geometry North of the estuary mouth extend the Sands of Forvie, the most extensive sand dune formation in Europe, which has been shown to have been a Stone Age settlement. Some of the dunes north of the estuary rise to a height of about and the lower ones to the south of the estuary rise to about in height. Proceeding from the mouth the estuary inland, there are initially beach sands at the margin, thence shingle beach and mudflats. Quickly, by the time of reaching the first roadway bridge crossing about up the estuary, there are mussel beds and marshy areas. At its widest point the Ythan Estuary is approximately wide. The verges of the upper reaches range from extensive mudflats to marsh and fen. Further upstream is a small island known as Geck Island () inaccessible to waders even at low tide, and which is