The Embanking of the tidal Thames is the historical process by which the natural River Thames, a broad, shallow waterway winding through malarious marshlands, has been transformed into a deep tidal canal. Today, over 200 miles of walls line its banks from Teddington down to its mouth in the North Sea; they defend a tidal flood plain where 1.25 million people work and live. The Victorian civil engineering works in central London, usually called "the Embankment", are just a small part of the process.
For a long time, who created the walls was a mystery. To embank a macro-tidal river seemed to call for a major civil engineering programme, yet none was mentioned in the historical record. For lack of a better theory, the works were attributed to "the Romans". The geologist and archaeologist Flaxman Spurrell was the first scientist to supply the currently accepted explanation. He realised that the land has been subsiding relative to the sea — hence, the tidal limit of the Thames has been moving upstream — but slowly enough for frontagers to adapt. Thus, the embanking of the Thames started at a time when the river was not tidal, or not strongly so. Only low embankments were needed at first; they were raised gradually as the land continued to sink and the waters to rise. In general, the Thames is narrower than it used to be.
Although the walls were created to win land from water, doing it had an unforeseen consequence, a "much more important thing to the country at large": the formation of a deeper Thames navigable by larger ships that could travel further inland, in which grew the largest port in the world.
The process was not a smooth one. If a wall was poorly maintained the river might break through, drowning the adjoining lands, sometimes for years at a time. Mishaps of that sort happened quite often. Legal disputes provide the first historical records of the embankment process and show it started before 1200. A special branch of the common law, marsh law, evolved to deal with such disputes.
It has been argued that land reclamation in the Thames contributed to the rise of independent farmers and the decay of the feudal system. Other political consequences were said to be two clauses in Magna Carta, and one of the declared causes of the English Civil War.
Much of present-day London is recovered marshland and considerable parts lie below high water mark. Some London streets originated as tracks running along the wall and yet today, are not even in sight of the river.