Page 142 - Sedimentology and Stratigraphy

9

Rivers and Alluvial Fans

Rivers are an important feature of most landscapes, acting as the principal mechanism
for the transport of weathered debris away from upland areas and carrying it to lakes and
seas, where much of the clastic sediment is deposited. River systems can also be
depositional, accumulating sediment within channels and on floodplains. The grain size
and the sedimentary structures in the river channel deposits are determined by the
supply of detritus, the gradient of the river, the total discharge and seasonal variations
in flow. Overbank deposition consists mainly of finer-grained sediment, and organic
activity on alluvial plains contributes to the formation of soils, which can be recognised
in the stratigraphic record as palaeosols. Water flows over the land surface also occur as
unconfined sheetfloods and debris flows that form alluvial fans at the edges of alluvial
plains. Fluvial and alluvial deposits in the stratigraphic record provide evidence of
tectonic activity and indications of the palaeoclimate at the time of deposition. Compar-
isons between modern and ancient river systems should be carried out with care
because continental environments have changed dramatically through geological time
as land plant and animal communities have evolved.

9.1 FLUVIAL AND ALLUVIAL SYSTEMS zone, where sediment is deposited in the river chan-
nels and on the floodplains of a fluvial system or on
Three geomorphological zones can be recognised the surface of an alluvial fan. These three components
within fluvial and alluvial systems (Fig. 9.1). In the are not present in all systems: some may be wholly
erosional zone the streams are actively downcut- erosional as far as the sea or a lake, and others may
ting, removing bedrock from the valley floor and not display a transfer zone. The erosional part of a
from the valley sides via downslope movement of fluvial system contributes a substantial proportion of
material into the stream bed. In the transfer zone, the clastic sediment provided for deposition in other
the gradient is lower, streams and rivers are not sedimentary environments, and is considered in
actively eroding, but nor is this a site of deposition. Chapter 6: the depositional zone is the subject of this
The lower part of the system is the depositional chapter.