Page 155 - Fundamentals of Geomorphology

138 STRUCTURE

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