Geoscience Reference
In-Depth Information
can become reactivated. The dominant wind direction when these dunes were active
was from the north-west during times when the large ice caps over North America
may have partly diverted the westerlies (Kutzbach and Wright, 1985 ). Factors con-
tributing to sand movement during glacial times include extreme cold, strong winds,
depleted or absent vegetation and a supply of available sand from older dunes. The
low concentration of atmospheric carbon dioxide during glacial maxima, amounting
to 180-200 ppmv, would have also contributed to the sparse plant cover (Warren,
2013 , p. 161). Both the Nebraska Sand Hills and the dunes of the Gran Desierto in
north-west Mexico have luminescence ages indicating that they were already active
during the Middle Pleistocene some 300,000 years ago (op. cit., p. 163).
20.7.3 River fluctuations
Rivers rising in arid and semi-arid regions have a much more erratic flow regime than
rivers in humid regions, with long intervals of low flow punctuated by occasional
extreme floods (see Chapter 10 ). Changes towards a more humid climate resulting
in a more complete plant cover can lead to changes in river channel pattern (e.g.,
from braided to meandering) and in the type of load carried (e.g., from coarse traction
load to fine suspended load). As a result, the sediments and fossil soils exposed in
alluvial fans and river terraces can provide a rough guide to the type of environment in
which they were transported and laid down. For example, during the Late Pleistocene,
the rivers of the Atlantic Coastal Plain were well beyond glacial limits, and reveal a
change from a previous meandering pattern to a braided pattern that was synchronous
with local dune formation (Leigh et al., 2004 ). The dunes were derived from fluvial
channel sands and point bars. In desert environments, it is often necessary also to
take into account possible sediment inputs to the rivers from sand dunes and wind-
blown dust. The highest sediment yields in the rivers of North America are to be
found in semi-arid regions (Langbein and Schumm, 1958 ), and on a more global
scale, a second peak occurs in the seasonally wet or monsoonal tropics (Douglas,
1967 ). The peak in semi-arid areas occurs because of sparse plant cover and the high
erosive impact of the rain. In arid areas, there is not enough rain for sustained run-off
and fluvial erosion, and in humid areas, the dense plant cover protects the soil from
raindrop impact and soil loss from overland flow (see Chapter 10 ). In the monsoonal
tropics, the highest rates of soil loss occur at the start of the wet season, when heavy
convectional downpours fall on bare, unprotected soil (see Chapter 10 ).
The situation becomes more complicated when the rivers are located downstreamof
large lakes that were formed by glacial damming, because the sudden release of water
caused by a breach in the ice dam can have catastrophic effects. In Iceland, such a
glacial outburst flood (or Jokulhlaupt ) can result in peak flows of
50,000 m 3 /second.
The 'Channeled Scablands' of Washington were formed by a similar sudden release
of water from an ice-dammed lake, Glacial Lake Missoula (Baker, 1978 ; Baker and
>
Search WWH ::




Custom Search