When evaluating which lessons are best to take away from study of the river, we must first ask ourselves "in which sense do we mean, philosophical lessons or environmental and scientific lessons?" As this article is currently classified under sciences, water and oceanography, I will adhere , as best as possible to commonly used oceanographic principles and terminology. Where appropriate I will also attempt to provide links an references to all concepts used. None the less, this is not a scientific article. This is an article about the river as a focus of human activity, and the river, in particular the estuary, as a focus of biological activity.
One of the first things we can observe in the study of human geography, or demographics, is that people are not evenly spread over the land, they concentrate at locations where a great river meets the sea. This is a principle today with cities in the U.S. such as New York and New Orleans straddling great rivers, as it was thousands of years ago in such historic cities as Cairo, Egypt and its legendary Nile River. The explosion of marine life, in particular rich fisheries gives rise to a large resident human population. Of course with a large resident human population comes inevitable stress on the natural environment. Thus, when we think of the location where a major river empties into the sea, we usually think of a shore lined with busy factories and port equipment. But in an oceanographic sense, there is a different type of factory operating in the estuary itself. It is a carbohydrate factory that we usually refer to as primary production which in turn provides the raw materials for a protein factory, more commonly known as fish.
Although in modern terms the may be many reasons for a cities existence, in historical terms major coastal cities are dependent on fishing and associated fruits of the sea such as travel and recreation. An estuary as it is called, accounts for the bulk of sea life as it is easily available to man. There are other productive areas in the ocean, for example where currents force water over underwater mountains and cause upwelling of deep water to the surface, an example being the Grand Banks, but these tend to be offshore and relatively difficult to harvest. Most of the ocean is a desert, at least in terms of supporting great amounts of primary production.
By primary production, we mean harvesting of the suns energy by means of photosynthesis, and conversion of nutrients into carbohydrate that can provide a foundation for a food chain. In because the pigment which powers the process of photosynthesis, chlorophyll, is green, we can map primary production in the ocean. NASA has done this with their Colors of Life Project. NASA has used satellites, first Coastal Zone Color Scanner, and again with SeaWIFS to create color and animated maps of primary production in the ocean. What we learn from these data collections and preparations is that where rivers meet the sea, in this example the Mississippi and the Amazon, explosions of life occur, and in fact these River/Sea interfaces are the engines that power the chain of life in the sea, on which we have historically been dependent. Thus we see that the uneven distribution of human population that we observe on maps often mirrors the uneven distribution of primary production we see in the ocean.
We should ask the question "what exactly is it about the estuary that makes it so productive" in order to understand it and preserve and protect it from the impact of human populations which inevitably form on large estuaries. One if the first things we note is that rivers contain a large amount of suspended nutrients which they pick up from both natural and man made sources. Most importantly, nutrients that some from the decomposition of plant matter tend to drop the pH of the water, or make it more acidic. Water that is more acidic both contains more nutrients in dissolved form, which can be used directly by photosynthetic organisms, and at low pH, dissolved carbon dioxide becomes more available for primary production.
As sea water mixes with river water in an estuary, a number of things happen. First of all, the sea is buffered at a pH of about 8.1, so nutrients required by autotrophs ( primary producers ) find them to be less available as they clump, or form flocks, and fall out of suspension to the bottom. Also at the alkaline pH of sea water, carbon dioxide in the water becomes bicarbonate ion, and as such, it the primary carbon source for primary production is no longer available. So at full sea water salinity and pH, the sea becomes a desert, at least in terms of primary production. As I mentioned previously, there can be exceptions. The ocean pH is not uniform along the "water column", or vertical dimension of the sea, and deep water characteristically has a dropped pH due to the effects of decomposition. If deep water is forced to the surface, due to some type of upwelling event, there can be a burst of primary production as nutrients and carbon are again recycled into carbohydrate by primary producers.
It goes without saying that estuaries are of great importance to man for reasons other than fishing. Where the great rivers meet the sea are also where ports are placed to receive cargo from across the ocean and transport it up river. Likewise, products from up the river are transported down the river to be carried to foreign lands. But the lesson is that regions which are likely to have the greatest environmental impact from human activities are also those which tend to be the most important to the natural world. When making decisions about use of land around estuaries we should keep in mind that for environmental and chemical reasons beyond our control, these are the engines of the carbon cycle as it exists in the ocean, and is an important link in the food chain upon which we all depend.
 N.A.S.A The colors of Life Retrieved from http://www.gsfc.nasa.gov/gsfc/earth/pictures/2001/0327colors_of_life/carbon.htm