Flowing water at the land surface - Fundamentals of the Physical Environment - page 459

Stable pool-and-riffle series develop spacings five to seven times channel width.
Meander wavelengths are about one order of magnitude larger than channel width.
Alternating bars migrate to form central bars or riffles , and point bars develop in slack
flow on the inside of meanders. Sinuosity develops characteristic channel and bank forms
with their own terminology and parameters (see Figure 14.21). New studies suggest that
meandering channels are metastable and that wholly new forms appear if threshold
conditions change significantly. This is apparent with Earth's more dynamic, high-
discharge rivers but it is also observed in historical studies of seemingly tame streams in
Britain such as the modest river Dane in east Cheshire. A 10 km stretch, now conserved
as a Site of Special Scientific Interest (SSSI), with a channel width of about 15 m, a mean
daily flow of 3 m 3 sec −1 and a moderately 'flashy' regime, is known to undergo
episodically very rapid rates of meander evolution on centennial time scales.
BRAIDED CHANNELS
Low flows divide into separate streams around central bars, whilst at higher flows central
bars are dissected and point bars cut off by the development of an inside chute . The bars
are normally resubmerged and single-channel flow is restored at mean flow levels, but for
a time the channel was braided . Persistent braiding represents unstable channel
conditions. It usually occurs in steep rivers experiencing high discharge and carrying
large-calibre high sediment loads and in glacial or arid environments subject to rapidly
changing water and sediment discharges. More stable, high-discharge mountain channels
braid on entering the piedmont zone. In effect, river flow is overloaded with bed load
beyond the competence of all but the highest discharges. Large sediment volumes are
dumped as soon as discharge or channel slope falls significantly. This drastically
increases channel roughness, giving the highest Reynolds number. Flow subsides into a
series of distributary channels with lower total roughness, and individual bars migrate
downstream. Rivers draining the Himalayas and New Zealand's Southern Alps may
spread 5-20 km wide with width-to-depth ratios of 250-300:1 or more. Where vegetation
stabilizes bars exposed for long intervals between submergences, redefining them as
islands, the braided channel is said to be anabranching .
THE FLOOD PLAIN
The third and largest scale of dynamic sedimentary environments is the flood plain ,
which loosely describes the valley floor prone to episodic over-bank discharge. In
another departure from classic fluvial landsystems, narrow flood plains with chemically
and texturally raw sediments occur in pockets within mountain catchments. Lowland
flood plains of more mature sediments are far more horizontally extensive, developed by
lateral accretion in meandering or braided rivers and vertical accretion through over-bank
discharge. Both forms usually occur together, and developmental history is seen in the
array of abandoned channel forms on flood plain surfaces and strati-graphic exposures
10 1-3 m thick in palaeo-environmental sediments. Cut - and - fill structures are indicative of
lateral accretion. They form a level floor as meandering channels cut into older deposits
and rework them as backfill into abandoned channels, mostly as bed load. New channels
form more dramatically through avulsion , where rivers break through old channel