A brief introduction to... Estuaries

www.thames.me.uk

Here goes! As promised, my first brief introduction to... I do need to clarify why I didn't post anything yesterday. This is because it was the last exam of my degree yesterday so I thought that was a tad more important! Enough about me, on to estuaries.

An estuary can be considered the connective zone between a river environment and the coast, and is subject to fluvial and coastal processes to varying degrees. The best definition of an estuary can be found in John Pethick's Introduction to Coastal Geomorphology (1984):

"...an inlet to the sea where a river comes into contact with seawater and is subject to tidal currents".

 The most interesting thing about estuaries from a physical standpoint is the drastically different depositional environments caused by inlet scale and morphology, tidal range and river discharge (with respect to inlet size and shape).

An estuary subject to a small tidal range (microtidal) and a dominant river discharge results in an interaction between saline sea water and fresh land drainage known as a "salt wedge". A salt wedge is formed when river water flows over the top of the denser saline seawater, with little mixing of the water due to a low tidal range. The majority of sediments in these estuaries are derived from land, being carried by the dominant flow of the river.

A greater tidal range (macrotidal) in an estuary with a river discharge proportional to the scale of the bay (in sync with the geomorphological setting) is subject to partial mixing of the saline seawater and the fresh river water. Deposition in these estuaries is predominantly seaward derived, with coarser sediments deposited at the mouth of an estuary, grading to finer coastal sediments being washed up the estuary into the head of the bay by tidal currents. The Thames Estuary is a good example of this, with fine muds deposited upstream; see the beautiful picture of the river Thames and its estuary above.

If the tidal range is great (macrotidal) but the river feeding into the estuary is a smaller discharge than required to produce the inlet (ie. a river follows the path of a previous, larger, glacial river for example), the river and seawater act in a unique way. Due to the Coriolis Effect, river discharge flows along the right hand bank of the estuary (looking out to the sea) and deposition of fluvially derived sediments takes place. By the same principal, tidal currents push water upstream along the left bank, depositing seaward derived sediments along this bank up to the head of the inlet.

From an ecological point of view, estuaries are a unique habitat hosting a massive variety of bird species especially. See www.deeestuary.co.uk for an ecological perspective on the importance of estuaries. www.thamesweb.com is a particularly good website on the aforementioned Thames Estuary, with emphasis on considering all stakeholder perspectives.