The fluvial history of the lower Vaal River catchment
Date
2009-05-28T11:37:23Z
Authors
Gibbon, Ryan James
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
Cosmogenic nuclide burial dating was used to determine the age of ancient
alluvial terrace deposits in the lower Vaal River catchment, and thus provide
an absolute fluvial chronology. A poor chronological framework has been the
primary shortcoming of previous fluvial history studies as there was no accurate
way to evaluate or correlate the proposed driving mechanisms with the fluvial
events themselves. Cosmogenic nuclide burial dating has shown that there have
been three periods, each of both river incision and alluvial gravels and fines
aggradation, in the Pliocene. Burial dating determined that periods of bedrock
incision have occurred relatively rapidly over short periods of time, following
lengthy periods of net aggradation. It has been proposed that the lower Vaal
River catchment provided the ideal situation for rapid bedrock incision to pro-
ceed sporadically. The Vaal River in its lower reaches is largely confined to
pre-Karoo valleys that are filled with soft, easily erodible Karoo rocks. When
these rocks are at least partially exposed along the river channel/valley, rapid
bedrock erosion and incision occurs through the action of saltating resistant
clasts abrading the channel. It has been argued that these incision events oc-
cur only when suitable climatic conditions develop that allow for the complete
erosion of alluvial gravel deposits lining the Vaal River channel/valley (i.e. fre-
quent and large formative discharges are needed to achieve this). Periods of
gravel aggradation have been attributed to an increase in coarse sediment sup-
ply, primarily through the erosion of older alluvial deposits in the valley during
periods of reduced vegetation cover. Periods of fine sediment aggradation are
the net result of a reduction in the capacity of formative discharges, a direct
result of a drying climate. It is thus apparent that climate change has played
the most important role in controlling the fluvial evolution of the Vaal River.
Climate controls fluvial events by determining the Vaal River’s net transport
ability. Various combinations of changes in transport and erosion potentials,
in both the river and surrounding landscape, have resulted in varying fluvial
outcomes. In conclusion, the fluvial evolution of the lower Vaal River catch-
ment is unique due to a number of aspects. The structure of the pre-Karoo
surface and readily available resistant clasts from older terrace deposits and
tillite, in conjunction with climate change, have proved crucial in shaping the
Vaal River’s fluvial history.