Geology Reference
In-Depth Information
Earth's crust. A little later, and they attributed terraces
in glaciated and unglaciated regions to climatic change
(Davis 1902; Gilbert 1900; Johnson 1901). William
Morris Davis (1902) posited that the slope of the long
profile of a river reflects a balance between the ero-
sion and transport of sediments, and believed that the
volume and nature of the sediment load are adjusted
to climate. A change from a humid to an arid cli-
mate, he surmised, would cause river long profiles to
steepen and aggradation to occur in valleys; whereas a
change from an arid to a humid climate would cause
river long profiles to become less steep and trenches to
form in valleys. Later workers were divided as to the
relative importance of, on the one hand, flood character-
istics and, on the other, sediment supply in explaining
the form and sedimentology of alluvial channels and
floodplains. Ellsworth Huntington (1914) opined that
valley alluviation in the south-west USA occurred during
dry episodes when vegetation was scanty and sediment
yields were high; and, conversely, degradation (chan-
nel entrenchment) occurred during wet episodes when
vegetation was more abundant and the sediment load
lower. In contrast, Kirk Bryan (1928) held that chan-
nel entrenchment in the south-west was associated with
periods of prolonged drought and occurred because the
much-reduced vegetation cover during long dry episodes
gave large floods. In turn, the large floods initiated
entrenchment, the trenches then expanding upstream.
Ernst Antevs (1951) endorsed this view. Taking yet
another tack, C. Warren Thornthwaite and his associates
(1942) attributed trenching over the last 2,000 years not
to major climatic shifts, but to changes in the intensity of
storms.
A modern review of the response of river systems
to Holocene climates in the USA argued that fluvial
episodes in regions of varying vegetation cover occurred
roughly at the same times, and that the responsiveness
of the rivers to climatic change increased as vegeta-
tion cover decreased (Knox 1984). Alluvial episodes
occurred between roughly 8,000 and 6,000, 4,500 and
3,000, and 2,000 and 800 years ago. Before 8,000 years
ago, changing vegetation and rapid climatic warming
caused widespread alluviation. The magnitude of this
alluvial episode generally rose to the west in parallel
with increased drying and increased vegetation change.
Between 8,000 and 7,500 years ago, erosion broke in
upon alluviation. Although of minor proportions in the
East and humid Mid-West, this erosion was severe in
the South-West. For the next 2,000 years, warm and
dry conditions in the southern South-West and parts of
the East and South-East (caused by the persistent zonal
circulation of the early Holocene epoch) led to a slow-
ing of alluviation in all places except the South-West,
where major erosion of valley fills occurred. Although
the South-East was warm and wet at the time, it did
not suffer erosion because forests were established. From
6,000 to 4,500 years ago, all the Holocene valley fills were
eroded, except those in the South-West, where alluvia-
tion continued. The extensive erosive phase resulted from
a climatic cooling that improved the vegetation cover,
reduced sediment loads, and promoted trenching; and
from the circulation of the atmosphere becoming increas-
ingly meridional during summer, so bringing higher
rainfall and larger floods. The South-West was untouched
by the erosive phase because the climate there became
more arid, owing to the northward displacement of the
subtropical high-pressure cell. Between about 4,500 and
3,000 years ago, the rates of erosion and deposition
slackened but were high again in many regions between
3,000 and 1,800 years ago. The nature of the intensi-
fication of erosion and deposition varied from place to
place. In the northern Mid-West, very active lateral chan-
nel migration with erosion and deposition took place.
On the western edge of the Great Plains, alluviation
occurred at many sites. In the southern Great Plains of
Texas, erosion and entrenchment were rife. The inten-
sity of fluvial activity then died down again and stayed
subdued until 1,200 to 800 years ago, when cutting, fill-
ing, and active lateral channel migration occurred. From
800 years ago to the late nineteenth century, a moder-
ate alluviation took place, after which time trenching
started in most regions. A lesson to be learnt from this,
and from other alluvial chronologies in other parts of
the world, is that the response of the fluvial system to
climatic change may not be synchronous, varying from
region to region, partly owing to regional variations of
climate and partly to thresholds within the fluvial system
itself.
