Geoscience Reference
In-Depth Information
next 100 years, greater erosion followed, and sedimentation rates peaked in 1850 at about
35 millimetres per year. In more recent times, since 1920, urbanization - protecting soils -
and the building of dams - blocking the delivery of sediment - have combined to reduce
erosion and sedimentation by an order of magnitude. The sedimentation rate has been re-
duced nearly to the background conditions that prevailed in the pre-European settlement
era.
Pollen from surrounding plants is often found in abundance in fluvial sediments, and the
analysis of pollen can yield a great deal of information about past conditions in an area. The
type of vegetation can be modified by all sorts of factors, including human interference as
in the Bush River basin example, but also for entirely natural reasons such as a change in
climate or soil conditions. Very long sediment cores taken from lakes and swamps enable
us to reconstruct changes in vegetation over very long time periods, in some cases over a
million years and more. Because climate is a strong determinant of vegetation, pollen ana-
lysis has also proved to be an important method for tracing changes in past climates.
An important study of a 250-metre-long core from the bed of Lake Biwa in Japan, for in-
stance, showed changes in pollen over about the last 430,000 years, a period in which five
glacial-interglacial cycles could be recognized. During glacial periods, pollen from pine,
birch, and quercus (or white oak) trees was dominant, indicating a climate that was cool and
temperate, tending towards subarctic. During interglacial periods, by contrast, high pollen
values for species typical of a warm-temperate climate were found, including broad-leaved
trees such as the deciduous Lagerstroemia (or Crape myrtle) and the evergreen Castanopsis
(a type of beech).
Other evidence for environmental change can be detected in larger-scale elements of the
landscape. The floodplains of many modern rivers, for example, bear traces of former chan-
nels, so-called 'palaeochannels', which are different in scale and/or form from the current
river. If a palaeochannel is buried beneath more recent sediments, it was probably formed
by a river that flowed towards a lower base level, indicating that local sea level or lake level
has subsequently changed. River terraces that are found in many river valleys are thought
to reflect fluctuations of climate, their formation having been driven by the direct and in-
direct influence of temperature and precipitation on fluvial activity.
Water power
