LANDSCAPES OF
FLUVIAL AND EOLIAN ORIGINS
This laboratory examines landscapes produced by fluvial and
eolian (wind) action.
Streams play a major role in shaping landscapes in humid-climate
terranes, where there is sufficient run-off water to create constantly-flowing
perennial streams.
Streams and the wind together play a major role in shaping landscapes
in deserts, where long periods of aridity and wind action
are rarely interrupted by torrential rainfalls that create ephemeral
streams.
STREAM EROSION
Streams erode alluvial valleys into the land surface by downcutting
into their valley floor and laterally eroding and widening their
valley walls.
Fluvial landscapes evolve progressive through three stages of erosion:
youth, maturity, and old age.
Each stage is distinguished by the:
- sinuosity and gradient of stream channels
- relative roles of downcutting and lateral erosion
- cross-sectional shape of the stream valley
Youth is characterized by:
- rapid downcutting
- streams with straight channels and steep gradients
- deep V-shaped alluvial valleys with steep slopes and narrow
flood plains
Maturity is characterized by:
- the start of lateral erosion and valley widening
- moderately sinuous channels with lower gradients
- alluvial valleys with moderately-sloping walls and wider flood plains
Old age is characterized by:
- extensive lateral erosion and little downcutting
- very sinuous channels with very low gradients
- alluvial valleys with gently-sloping walls and a broad, low relief
valley floor (peneplain)
The evolution of an alluvial valley from youth to old age can be interrupted
by tectonic uplift at any time.
This increases the relief of a region, rejuvenates its streams
and restarts the sequence of valley evolution.
Rejuvenation creates erosive features such as:
- incised meanders: formed when sinuous mature and old streams
are uplifted and resume down-cutting
- terraces: uplifted and abandoned floodplains
Streams can also increase the areas of their valleys and drainage basins
by the processes of:
- headward erosion: the erosion and retreat of their upper drainage
divides
- stream piracy: occurs when headward erosion enables one stream
to capture the water flow from a second stream
STREAM DEPOSITION
The three major continental environments of stream deposition are:
- alluvial fans
- braided streams
- meandering streams
In addition, streams also deposit their sediment in river-mouth deltas
along the coasts of oceans and lakes (which we will examine next week).
Alluvial fans:
- cone-shaped accumulations of stream and gravity deposits at the bases
of mountains
- surface is characterized by a radial drainage patterns of distributary
channels
- distributary channels split into progressively smaller channels from
inner to outer fan
- flow velocity in distributary channels also decreases from inner to
outer fan
Braided streams:
- characterized by several channels and channel-bottom bars
- stream channels split and rejoin around channel bars to form a braid-like
pattern
- channels are straight to slightly sinuous, and wide and shallow in
cross-section
Meandering streams:
- consist of single meandering channel which flows through a broad, low-relief
floodplain
- channel erodes cut banks on outer meander bends and deposits
sandy point bars on inner bends and silty levees along its
banks
- lateral migration of channel-point bar-levee complex creates a ridge-and-swale
topography
- abandoned meander bends form oxbow lakes
DESERT STREAMS
Rainfall is rare in deserts, but when it does occur, it tends to be
rapid and torrential. This causes extensive erosion and deposition of sediments
by ephemeral streams that flow only during and immediately after
the rainfall.
Ephemeral desert streams consist of braided channels (called wadis,
washes, and arroyos) and low-relief floodplains (called sand
flats and mud flats).
They often flow into ephemeral lakes called playas that form
in grabens and other topographic lows on a desert floor.
Erosion by desert streams produces:
- inselbergs: isolated remnants of mountain ranges and ridges
separated by water gaps
- pediments: gently sloping erosion surface along mountain fronts
Deposition of desert streams produces:
- alluvial fans: that cover the pediments
- bajadas: broad mantles of coalescing fans
- wadi (channel) and sand/mud flat deposits
- playa-margin deltas
EOLIAN PROCESSES
Deserts are land surfaces with sparse plant cover.
The lack of vegetation is due to the fact that deserts are generally
arid for very long periods of time, and only rarely receive rainfall.
The lack of soil-binding vegetation allows the wind to easily erode
sediment and mold it into distinctive depositional landforms, the most
important of which are dunes.
EOLIAN DUNES
Dunes are migrating ridges of windblown sand. They migrate wherever
there is no plant cover to bind their sand to the desert floor. They are
stabilized by either topographic barriers or vegetation.
Dunes are asymmetrical in profile, with a gently sloping upwind (stoss)
side and steeply sloping downwind slip face that are separated by
a crest.
The most common types of dunes are:
- barchans: crescent-shaped dunes with horns that point
downwind
- tranverse: linear dunes with straight crests that are oriented
normal to the wind direction
- parabolic: crescent-shaped dunes with horns that point
upwind
- longitudinal: linear dunes with straight crests oriented parallel
to the wind direction
- star: complexly shaped dunes with many crests and slip faces
formed by shifting winds
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