Geology Reference
In-Depth Information
5
Centrifugal drainage
is similar to radial and occurs
where, for example, gutters develop on the insides
of meander loops on the tidal mudflats of coastal
north-west Queensland, Australia.
6
Centripetal drainage
has all streams flowing towards
the lowest central point in a basin floor. It occurs in
calderas, craters, dolines, and tectonic basins. A large
area of internal drainage lies on the central Tibetan
Plateau.
7
Distributary drainage
typifies rivers debouching
from narrow mountain gorges and running over
plains or valleys, particularly during occasional floods
when they overtop their banks. Many deltas display
a similar pattern of drainage (p. 341).
8
Rectangular drainage
displays a perpendicular net-
work of streams with tributaries and main streams
joining at right angles. It is less regular than trellis
drainage, and is controlled by joints and faults. Rect-
angular drainage is common along the Norwegian
coast and in portions of the Adirondack Mountains,
USA. Angulate drainage is a variant of rectangular
drainage and occurs where joints or faults join each
other at acute or obtuse angles rather than at right
angles.
9
Annular drainage
has main streams arranged in a
circular pattern with subsidiary streams lying at right
angles to them. It evolves in a breached or dissected
dome or basin in which erosion exposes concen-
trically arranged hard and soft bands of rock. An
example is found in the Woolhope Dome in Hereford
and Worcester, England.
the area are also orientated in a similar direction to the
bedrock joints. Both the bedrock channels and modern
river channels bear the hallmarks of tectonically pre-
designed landforms (Eyles and Scheidegger 1995; Eyles
et al
. 1997; Hantke and Scheidegger 1999).
Structural and tectonic features, such as joints,
faults, and lineaments (p. 144), may produce essen-
tially
straight rivers
, that is, rivers with limited meander
development (Twidale 2004). Joints and faults may
produce short linear sections of rivers, typically a few
tens of metres long. Longer straight rivers commonly
follow regional lineament patterns, an example com-
ing from central and northern Australia, where long
sections of several alluvial rivers, including the Finke
River, Georgina River, Thompson River, Darling River,
and Lachlan River, track lineaments in the underlying
bedrock. The Darling River, flowing over Quaternary
alluvium, follows a lineament in Palaeozoic and Mesozoic
bedrock between St George in south-east Queensland
and near Menindee in western New South Wales, a
distance of about 750 km.
Anomalous drainage patterns
Anomalous drainage bucks structural controls, flowing
across geological and topographic units. A common
anomalous pattern is where a major stream flows across
a mountain range when just a short distance away is an
easier route. In the Appalachian Mountains, north-east
USA, the structural controls are aligned south-west to
north-east but main rivers, including the Susquehanna,
run north-west to south-east. Such
transverse drainage
has prompted a variety of hypotheses: diversion, cap-
ture or piracy, antecedence, superimposition, stream
persistence, and valley impression.
Recent investigations by Adrian E. Scheidegger reveal
a strong
tectonic control
on drainage lines in some
landscapes. In eastern Nepal, joint orientations, which
strike consistently east to west, in large measure deter-
mine the orientation of rivers (Scheidegger 1999). In
south-western Ontario, Canada, the Proterozoic base-
ment (Canadian Shield), which lies under Pleistocene
glacial sediments, carries a network of buried bedrock
channels. The orientation of these channels shows a sta-
tistically significant relationship with the orientation of
regional bedrock joints that formed in response to the
mid-continental stress field. Postglacial river valleys in
Diverted rivers
Glacial ice, uplifted fault blocks, gentle folding, and lava
flows may all cause major
river diversions
. Glacial ice
is the most common agent of river diversions. Where it
flows across or against the regional slope of the land, the
natural drainage is blocked and proglacial or ice-dammed
marginal lakes grow. Continental diversion of drainage
