ABSTRACTS / RESUMES - Comitato Glaciologico Italiano

lake deposition, and deviations from these relationships through
time, provide a measure of the potential importance
of transport lag times, depositional censorship, and temporal
variations in erosion resistance in small, steepland catchments.
On-going work nearby in the 2,050 km 2 Waipaoa
catchment is intended to explore how these influences are
affected by processes active along larger, lower-gradient
channels.
GREGORY E. TUCKER 1, KELIN X. WHIPPLE 2
& RAFAEL L. BRAS 1
The impact of tectonic variations on sediment yield
and river basin morphology
1Department of Civil and Environmental Engineering, MIT,
Cambridge, MA 02139, USA
2 Department of Earth, Atmospheric, and Planetary Science, MIT,
Cambridge, MA 02139? USA
Tectonic uplift is an important control on both the
morphology of the uplifted landscape and in the sediments
that are shed from it. Qualitative interpretations of the relationship
between uplift, erosion, and sediment yield are
central to much of geological basin analysis, particularly in
regard to the reconstruction of tectonic history from basin
sediments. Quantitative interpretations of basin-fill sedimentary
packages, however, requires a mechanical understanding
of the relationship between uplift rate, sediment
yield, and drainage basin sculpture. Here we analyze this
relationship using a physically-based model of coupled hillslope
and channel evolution. In particular, we seek to address
the following questions:
1. What do predicted sediment-yield curves look like (e.g.,
what is the magnitude and timescale of response to a given
perturbation."), and how do they vary as a function of process?
2. What is the predicted relationship between relief,
uplift, and denudation rate, how does it vary with time,
and how does this relationship compare with sediment
yield data?
3. How do morphometric properties such as drainage
density vary with uplift rate, and how are these related to
the sediment yield response?
The model is based on the evolution equation
Bz/Bt = U - F[Q(x, y, t), S(x, y, t)] + H[S(x, y, t)]
where z is elevation, t time, U uplift rate, Q water discharge,
and S surface slope. Here, F[] is a fluvial erosion function
that takes the form kQIDS (representing detachment-limited
bedrock channels) or BkQ IDS n/Bx (representing
transport-limited alluvial channels). Several different processes
represented by the hillslope sediment production
and transport term H[] are considered, including soil production
through weathering, diffusive creep, dry landsliding,
and pore-pressure-driven shallow landsliding. An important
aspect of the model is its ability to track temporal
and spatial variations in soil thickness, and to differentiate
between erosion of soil and bedrock, which proceed at different
rates and by different mechanisms.
Dimensional analysis and numerical simulations indicate
that at steady state, where a dynamic balance between uplift
and erosion exists, relief within the fluvial portion of a catchment
should scale with uplift rate as R"JUlIn, where n reflects
the physics of channel erosion. If uplift and denudation
rates are approximately equal, this result also implies
that denudation rate scales with relief as D"JR", This result
applies to transport-limited as well as detachment-limited
fluvial systems, but only under the steady-state condition.
Numerical simulations afford the opportunity to examine
transient responses to system perturbations. Here we describe
responses to a single step function increase in uplift
rate (U). In addition, we consider only single catchments of
fixed size and shape, intentionally avoiding the complications
associated with lithologic variations, temporary tectonic
ponding of sediment, and stream capture. Simulations
run under a variety of conditions allow examination of how
sediment yield responses differ under different hillslope
process regimes (e.g., regolith creep vs. landsliding),
Some preliminary findings can be outlined. In simple detachment-limited
or transport-limited model runs, sediment
response curves are monotonic, increasing smoothly to match
the newly imposed uplift rate. Response times cales are
governed both by the fluvial system and by the operative
hillslope transport process. For instance, a more rapid response
occurs when hillslopes are dominated by mass movement
than when they are dominated by diffusive soil
creep. However, comparisons with runs involving more
complete process descriptions (e.g., tracking soil thickness)
emphasize the limitations of the simplified models. Explicitly
tracking soil thickness enables us to capture some of the
complex dynamics of channel-hillslope interactions that influence
runoff rates, drainage density, and rates of sediment
delivery to channels. Because sediment stored in soils on
hillslopes can be rapidly excavated as channels extend
upslope in response to, e.g., a sudden change in baselevel,
channel-hillslope interactions can dictate the form of sediment
yield response curves, which may deviate significantly
froin the monotonic increase predicted by simpler models.
KRYSTYNA TURKOWSKA
Role morphogenetique des depots du Plenivistulien
Moyen dans la Pologne Centrale; etudes detaillees dans
la vallee de la Mrozyca
Institut de Geographic Physique et de l'Amenagement
de 1'Environnement,
Universite de L6dz, llrue Sklodowska-Curie, 90 505 L6dz, Poland
La Mrozyca est une de petites rivieres descendant du Plateau
de £6dY vers la pradoline de Varsovie- Berlin. Dans sa
vallee, il y a deja un demi siecle, Jan Dylik a commence des
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