Flowing water at the land surface - Fundamentals of the Physical Environment - page 456

ephemeral river flow has exposed the massive structure of the
fan.
Photo: Ken Addison.
Figure 14.20 Morphology, material fluxes and development
of a lake system. Relative inputs-outputs of the water and
sediment fluxes (including dissolved load) are shown by the
size of the arrows. More sediment enters than leaves the
system, leading to its eventual infill, and outlet lowering
progressively lowers the water level.
sand-gravel fraction (0·06-60·0 mm) and above in straight channels but includes
substantial silt (0·002-0·006 mm) and even clay (< 0·002 mm) fractions in sinuous
channels. Not all this material reaches its final marine destination; the geological record
reveals substantial components of lithified terrestrial sediment in cratons or incorporated
in continental collisions. Alluvium is reassimilated into New Zealand orogens before
reaching the Pacific Ocean (see Chapter 25). For all that, unlithified alluvial sediments
are far more easily eroded than bedrock and facilitate channel adjustment to flow regime.
The flood plain environment also experiences dynamic changes at whole-channel and
flood plain scales, depositing and remobilizing soft sediments.
STRAIGHT AND MEANDERING CHANNELS
Meandering is the natural tendency for alluvial channels, although it was once thought to
reflect sluggish inability to maintain a more direct line in the 'senile' stage of the river.
Straight channels are uncommon, except in heavily regulated and 'channelized' rivers
where flood evacuation is a required aim. Ironically, this speeds water on to the next
downstream unprotected zone, and new trends in river management include meander
restoration (see box, p. 300). Straight channel segments carry low bed loads, compared
with meandering forms. Many natural channel segments may appear straight but the