|"Estuaries are coastal bodies of water, within which seawater is measurably diluted by fresh water derived from land drainage." is the "classical" 1963 definition of estuaries by Cameron and Pritchard. Others state that estuaries are the site where rivers debouch into the sea (or a lake). But what is the difference between estuaries and deltas? One helpful way to answer that question is to look separately at the "bucket" and its "contents". Like a bucket, estuaries as well as deltas consist of more or less solid sides and a bottom. The form of these boundaries is the domain of study of the geomorphologist, who studies landforms. The contents of the bucket is water in almost continuous motion, making it one of the topics of study of physical oceanography. The main motions are caused by gravity (river flow and tides - the latter due to diffferences in gravitational attraction between the Earth and the Moon in its orbit around the Earth) and by wind stress. Because of its varying content of both dissolved matter (salinity) and suspended matter, the water in the delta/estuary "bucket" is of considerable interest to marine chemists studying pollution and geologists/engineers studying sediment transport.|
|Whether a river entering into the sea forms an estuary or delta depends on the amount of sediment the river carries and the effectiveness of ocean waves and currents to remove that sediment. Because of the recent geologic history of sea level rise and transgression of the sea ("Global Warming" is nothing new to the geologist: it has been going on for well over 10,000 years, intermittently, since the last Ice Age), "base level" - the level to which rivers can erode their bed - has been changed causing rivers to be invaded by the sea at the same time that they provide less sediment. Human influences on this process include building of dams, which trap sediment. (PA article) and levees, draining of marshes, canalization and cutoffs, as well as urbanization (pavements and parking lots) industrialization (pollutants) and deforestation.|
|In the case of the Mississippi, its ample sediment supply and the relative protection from strong ocean currents and waves at its mouth caused its peculiar "birdfoot" delta shape, different from the classical Greek letter Delta characterizing the Nile delta. The Changjiang river delta - with Shanghai at its mouth - provides large amounts of silt and clays and has been notoriously mobile in its recent history. The "Dutch Delta" at the mouths of the rivers Rhine, Maas (Meuse) and Scheldt actually consists of a series of estuaries. In case of the Amazon River, its delta is mainly offshore, and the amount of fresh water is so overwhelming that the mixing with seawater takes place outside its "bucket", in the open sea. The intermittent Orange River in Southern Africa also has a submerged delta.|
Estuaries used to be typified by the Chesapeake Bay, the World's largest estuary,
but the recent discovery that a large meteor impact certainly contributed to its eventual shape makes this claim somewhat doubtful.
Furthermore, the Susquehanna was atypical in that is probably was the Worlds's largest river at the time when glacial smelting and sediment transport caused
obstructions in the Saint Lawrence Waterway as well as the Mississippi. Nowadays, the Chesapeake's sediment supply by its main river,
the Susquehanna, is greatly impeded by the many dams (also see this link) and reservoirs
in the Susquehanna. Nevertheless, its turbidity caused SAV (=submerged aquatic vegetation) and its associated blue crab population to
decrease dramatically, while pollution and some mysterious disease decimated the oyster harvest and lead to controversial
consideration of importing Chinese and Japanese oysters to replace local species.
Perhaps the Delaware is now a better "prototype" estuary because it has a simple mathematical shape, lacks dams, and cannot be dredged deeper till Pennsylvania and New Jersey can agree on the disposal of the dredge spoil, and resolution on the Fed-State controversy about the sudden sale of its Port of Philadelphia to Dubai ;o] I recall, however, that during a drought several decades ago, the lack of water was attributed to the fact that most of it sat on shelves in research labs waiting to be analyzed. Also, the propensity to use mathematical models led to a condemnation of my data, because the results of my current measurements and salinities did not agree with the model's prediction!
|The foregoing seems to be enough introduction to what I really wanted to talk about, the history of the study of estuaries. When I started my Ph.D. study of suspended sediment transport at the mouth of Delaware Bay, in 1968, the major papers produced mainly by US researchers were contained in George Lauf's conference volume "Estuaries". The main researcher, or Doyen of Estuaries, was Dr. Pritchard.|
(please click to enlarge)
The simplest model was the equation of continuity for the box model estuary:
R (riverflow) + I (inflow from the sea)
= O (outflow to the sea).
based on the 1940's formula in "The Oceans" by Sverdrup, Johnson and Flemming.
|Since Outflow generally consists of less saline and thus less dense water, it occurs along the surface (exception: hypersaline lagoons) so that, importantly, seawater intrudes along the bottom, (in the case of the Delaware carrying marine diatoms as far as 100 miles inland). This set the stage for some interesting consequences we researched, including that of the inappropriate disposal of dredge spoil and sewage sludge by dumping them in or near the mouth of Delaware Bay. Physical oceanographers call this: "primary estuarine circulation". I presume the "secondary" is due to the Coriolis Effect, causing the flow to deviate tot the right, so that -in the case of Delaware Bay, the outflow water hugs the right-hand (DE) bank and the inflow favors the NJ side.|
But what about "Tertiary Estuarine Circulation"? Did something like it exist? During a visit to my native Amsterdam in the late 1960's,
I visited the Library of the University of Amsterdam. Two things stand out in my memory:|
the correct answer is (d) because they are arranged by size !
I devoured it and thought it superior to any of the exclusively recent stuff I had read
in the USA by US authors. (though at a level with such Dutch estuarine researchers as Postma,
Groen and Terwindt.
I promised myself to translate it, but to-date have failed to do so. Now it may be too late 'cause I cannot find it'
I do, however, want to reproduce this figure, also used in my 1971 Ph.D. thesis: (right)|
This pattern is important because it results in stronger turbulence along the bottom during flood, thus stirring up (marine) sediments and carrying them landward.
|Canter Cremers, as early as 1921, recognized the practical problems of ineffective spoil disposal in the mouth of an estuary om account of estuarine circulation. The fact that in the USA and possibly in other countries, (although the UK may be an exception) this fact was unknown led to unconscionably high expenses incurred by dredge spoil and sewage sludge disposal.|
This is, of course, partly due to the "obscurity" and "before-its-time"-ness of Canter Cremer's article, published in the Dutch language.
Dutch is certainly not one of the "foreign languages" graduate students select
(my Prof Jerry van Andel ridiculed me for even thinking of asking to have me substitute it as one of the two required languages at Scripps in the early 1960's)
Yet there may be much more valuable material to be "mined" from foreign libraries (attention CIA ! ...).
I tried to learn Chinese for years and guest-lectured in Shanghai in the hope of "mining" what I thought would be thousands of years of Yangtze Kiang (Changjiang) records.
But again, I failed, using Denial (pun intended, The Nile) and blaming Emperor Qin Shi Huang (in 212BC) for "burning all books and killing all scientists";o]
Similarly, many tomes of the rich Arabic/Muslim/Middle East literature, e.g. of the Ptolemies, including records of the Nile, were burned with the Library of Alexandria.
Subsequent literature may well be at risk and become the victim of destructive mobs protesting Danish cartoons ;o]
Let's hope the WWW fares better!