Geoscience Reference
In-Depth Information
cales, such as hundreds to millions of years. The ripples in the water operate on much short-
er timescales: milliseconds to seconds.
It is also important to remember that, in general terms, as size increases, so too does the
complexity of factors influencing the landscape and the river running through it. Hence,
the small catchment of a first-order river channel, for example, may well occur on one rock
type and lie within one climatic region. A larger catchment is more likely to span several
rock types and climatic regions and is therefore more complex.
Types of river
The numerical ordering scheme detailed in the previous section is one of many attempts
to classify rivers. There is an enormous variety of different types of river, or 'fluvial' sys-
tem (from the Latin word
fluvius
, a river), when we extend our area of interest beyond
the river channel to include the entire drainage basin. Each river classification depends on
the perspective of the investigator and hence the aspect of greatest significance. A biolo-
gist may focus on the distribution of particular groups of organisms such as fish or aquatic
plants. Different species may be associated with different types of topography and geology,
for example, hence rivers may be placed in categories such as 'mountainous', 'upland',
'lowland chalk', 'lowland sandstone', and 'lowland and upland clay'. Others have used
selected chemical factors as a basis for classification. An example is pH, so rivers might
be classed as being strongly acid, slightly acid, or alkaline. An authority concerned with
nature conservation might combine all of these perspectives and more. A classification of
rivers in England, Wales, and Scotland based on vegetation communities devised by the
Nature Conservancy Council recognizes four main groups of rivers, ten types, and thirty-
eight sub-types.
Another simple way of categorizing rivers is by size. Some authorities prefer the word
'stream' when referring to rivers at one end of the size spectrum. A large or big river (both
words are in common use to signify the other end of the spectrum) is usually one with either
a large drainage basin, a long course, one that transports a large volume of sediment, or has
a great volume of water flowing in it. We have noted that there is a consistent relationship
between river length and drainage basin area, although not between the other variables due
to variations in basin geology, relief, and hydrology. Most people when asked to list the
world's largest rivers would come up with a similar list for their top 10 or 20, but a perfect
definition remains elusive.
