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 Fundamentals of Fluvial Geomorphology and Channel Processes
An example of the improper use of empirical relationships was provided by Mark
Twain in Life on the Mississippi (Clemens, 1944). In his wonderfully sarcastic manner, he
describes Mississippi River cutoffs of which he had knowledge. Therefore, he developed an
empirical relationship to predict the eventual length of the Mississippi River. He eloquently
describes the modeling process:
"Now, if I wanted to be one of those ponderous scientific people,
and "let on" to prove what had occurred in the remote past by what had
occurred in a given time in the recent past, or what will occur in the far
future by what has occurred in late years, what an opportunity is here!
Geology never had such a chance, nor such exact data to argue from! Nor
"development of species," either! Glacial epochs are great things, but they
are vague - vague. Please observe:
"In the space of 176 years, the Lower Mississippi has shortened
itself 242 miles. That is an average of a trifle over one mile and a third per
year. Therefore, any calm person, who is not blind or idiotic, can see that
in the Old Olitic Silurian Period, just a million years ago next November,
the Lower Mississippi River was upwards of 1,300,000 miles long, and
stuck out over the Gulf of Mexico like a fishing rod. And by the same
token, any person can see that 742 years from now the Lower Mississippi
will be only a mile and three-quarters long, and Cairo and New Orleans
will have joined their streets together, and be plodding comfortably along
under a single mayor and a mutual board of aldermen. There is
something fascinating about science. One gets such wholesale returns of
conjecture out of such a trifling investment of fact."
The primary point of this delightful sarcasm is that we should not fall into the trap of
attempting to plan a project based on "...wholesale returns of conjecture out of such a trifling
investment of fact." Empirical relationships can be very useful. We cannot be certain that
New Orleans and St. Louis will have a common Board of Aldermen on September 13, 2604;
however we must be certain that the data from which the relationship was developed is valid
for the project location, for the scale of the project, and that the relationship makes physical
sense in application to the project.
2.1.6 CHANNEL CLASSIFICATION
Several primary methods of river classification are presented in the following
paragraphs, and these methods can be related to fundamental variables and processes
controlling rivers. One important classification is either alluvial or non-alluvial. An alluvial
channel is free to adjust dimensions such as size, shape, pattern, and slope in response to
change and flow through the channel. The bed and banks of an alluvial river are composed
of material transported by the river under present flow conditions. Obviously, a non-alluvial
river is not free to adjust. An example of a non-alluvial river is a bedrock controlled channel.
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