ABSTRACTS / RESUMES - Comitato Glaciologico Italiano

flows, where flow competence increases on riffles and decreases
in pools, fines are winnowed laterally into downstream
pools where they are deposited and stored until the
rising limb of the next high flow. At this stage according to
theory pools become more «competent» and previously
deposited material is re-entrained. Field experiments conducted
on the River Rede an upland gravel-bed river in
Northumberland, UK, suggest however that fine sediments
interact with the pool-riffle sequence in a very different
manner. The natural flux of fines and the transport of an
artificially-introduced magnetic tracer were monitored through
the reach using an array of basket traps in conjunction
with freeze coring. Coupled with hydraulic information,
this enabled the production of a three-dimensional
picture of fine sediment development over time. Freezecore
data did not pick up higher concentrations of tracer
in pools as may have been expected; instead it appears that
deposition occurs upon riffles, particularly on the channel
peripheries and over point bar surfaces on the falling limb
ofhigh flow events. Fines are principally stored between
the interstitial spaces in the topmost layer of the sub-surfacesediments
(as revealed from freeze-core evidence), and
also as lenses on the lee-side of armour layer particles. There
was no evidence to suggest that fines were winnowed
from riffles into pools during waning flows. A conceptual
model describing behaviour of fine sediment transport in
upland pool-riffle systems is put forward.
JOHN D. MILLIMAN
Fluvial sedinment discharge to the sea
and the importance of regional tectonics
School of Marine Science, College of William and Mary,
Goucester Point, VA 23062, U.S.A.
The geomorphic/tectonic character of the drainage basin
and the actual basin area have first-order controls on the
sediment discharge of most rivers. Topographic control
(which in fact serves as a Surrogate for tectonic character
of the basin), however is strongly dependent on the erodability
of the substrate - younger sedimentary rocks being
more erodable than older crystalline rocks. Climate (particularly
precipitation) and human impact can play important
roles, often explaining deviations from the load predicted
on the basis of topography and basin area alone.
Suspended sediment (SS) yield of mountainous rivers increases
5 - to 9 - fold for every order of magnitude decrease
in basin area, the result of decreased storage capacity,
steeper gradients, and greater susceptibility to episodic
events such as floods and landslides. As a result, a large
percentage of the SS discharged to the ocean comes from
small mountainous rivers, which generally discharge onto
active margins. In contrast, a substantial part of the fluvial
sediment load of large rivers, which mostly discharge to
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passive margins, may be deposited on subsiding lowlands
or deltas; as a result, the actual sediment discharge to the
ocean may be considerably less for some large rivers than
cited in the literature. We therefore have probably overestimated
the sediment discharge of large rivers to the
ocean at the same time we have underestimated the discharge
of smaller rivers.
Dissolved sediment (DS) yield of mountainous rivers with
moderate to high runoff also increases with decreasing basin
area, out at a lesser rate than for SS. DS yield is demonstrably
lower for river basins with low runoff. While DS
yield increases with decreasing basin area, its increase is
less than that for SS. The resulting high SS/DS ratio in
smaller mountainous rivers presumably reflects the decreased
effect of chemical weathering due to the shorter period
of sediment storage between erosion and discharge to the
ocean.
Evolution of the river basin also can cause changes in fluvial
sediment discharge. For instance, as a river progrades
and (eventually) merges with other rivers. the SS yield
should decrease and DS should increase, a situation not
unlike that expected for rivers draining denuded mountains.
HUGH H. MILLS
Abandoned bedrock meanders used for the study
of hilIslope evolution
Department of Earth Sciences, Tennessee Technological University,
Cookeville, Tennessee 38505, U.S.A.
Evolution of hillslopes on resistant bedrock takes place so
slowly that direct observation of change in most cases is impossible.
Instead, it is necessary to order modern-day hillslope
profiles according to their relative age, and then consider
their forms to represent stages in a developmental sequence.
In the unglaciated Appalachians and Interior Plateaus
of southeastern North America, landscapes are poorly
dated, and finding a chronosequence of hillslope profiles is
difficult. One opportunity to do this is provided by incised
meandering streams near the western margin of the Eastern
Highland Rim, Tennessee, U.S.A. Incision below the surface
of the plateau is about 100 m, and valley walls are underlain
mainly by the chert-dominated Ft. Payne Formation.
These streams show «ingrown» meanders, characterized by
gentle slip-off slopes on the inside of the meanders and
steep undercut slopes on the outside. Some of these meanders
have been abandoned when stream erosion cut through
the narrow neck of the meanders. The floors of these
cutoff meanders range in height from 2 m to as much as 43
m above the modern stream level (henceforth abbreviated
«Ash». Based on regional denudation data and one measurement
elsewhere of stream incision rate, streams are incising
somewhere between 10 and 60 mm/ka, so that the age