A tidal creek, tidal channel, or estuary is the portion of a stream that is affected by ebb and flow of ocean tides, in the case that the subject stream discharges to an ocean, sea or strait. Thus this portion of the stream has variable salinity and electrical conductivity over the tidal cycle. Due to the temporal variability of water quality parameters within the tidally influenced zone, there are unique biota associated with tidal creeks, which biota are often specialised to such zones.
Creeks may often dry to a muddy channel with little or no flow at low tide, but often with significant depth of water at high tide.
In British English and in many other countries in the Commonwealth, as well as some parts of the United States (near the Chesapeake Bay, parts of New England), a creek is a tidal water channel. In the tidal section of the River Thames in London, the names of the rivers that flow into it all become Creeks for the lower section that is tidal; thus, for example the River Lea becomes Bow Creek in its tidal section. In parts of southwest England and Wales, the term 'pill' is used, and is found in placenames such as Huntspill.
On the India and Pakistan borders the term also applies to the salt water inlets enclosed by mangroves. Creeks are found dispersed all along the Indian coast. In the Florida Keys, a creek is a narrow channel between islands.
There are thousands of examples of tidal creeks throughout the world. A few specific ones are: