Geoscience Reference
In-Depth Information
higher elevations. Thus in Lahontan the more continuous vegetation cover reduced the generation
of runoff and sediment production, leading to only rare debris-flow events associated with the
winter storms. In contrast the sparse, discontinuous vegetation of Mojave allowed greater runoff
generation from the winter rainfall, and associated enhanced overall sediment erosion and delivery
in the form of debris flows from the winter storms. Incision and streamflow were probably
generated from the high intensity summer storms, which generated more effective runoff.
With increasing aridification into the Holocene, climate and vegetation became similar between
the two areas and sediment production and delivery differences became less pronounced. The
lakes dried up and in most cases the reduced vegetation cover associated with the climate
change enhanced effective runoff generation, leading to erosion of slopes and fluvial trenching
of the upper parts of fan systems in both areas. This has led to the progradation of many of the
fan systems, partly encouraged by associated drops in the lake (base) levels as the lakes dried up
(Harvey et al. 1999).
Relevant reading
Harvey, A.M., Wigand, P.E. & Wells, S.G. (1999) Response of alluvial fan systems to the late Pleistocene to
Holocene climatic transition: contrasts between the margins of pluvial Lakes Lahontan and Mojave, Nevada
and California, USA.
Catena
36
, 255- 81.
Jannik, N.O., Phillips, F.M, Smith, G.I, et al. (1991) A
36
Cl chronology of lacustrine sedimentation in the
Pleistocene Owens River system.
Geological Society of America Bulletin
103
, 1146 -59.
Alluvial fans may be affected by changes in
climate, where changes in process are affected.
For example, where climate either limits or
enhances the weathering processes within the
catchment the water to sediment ratio of the
dominant fan process may change. Similarly,
changes in the amount of effective runoff gener-
ated may again change the water to sediment
ratio and thus modes of sediment transport
and accumulation (Case Study 5.2). Streamflow
processes require less of a gradient for trans-
port than debris-flow fans, so a switch from
debris flow to streamflow may lead to fan-head
trenching. If the entire fan becomes trenched,
the mountain supply catchment will then be
able to supply sediment to the main basin area,
altering basin sediment routing. Alternatively
climate may control lake-levels in arid environ-
ments (Case Study 5.2). As with tectonic con-
trols, lake-level fluctuations can control alluvial
fan morphology and sedimentary architecture
by controlling the accommodation space for
alluvial fan development. Lake-level rise will
typically lead to retrogradation of the alluvial
fan system, backfilling into mountain embay-
ments. Viseras et al. (2003) demonstrated that
this is associated with a reduction in the size of
feeder channel. In these cases the fan morpho-
logy most strongly reflects the morphology of
the mountain embayment in which the fan was
developed, rather than the nature of the catch-
ment area. This reflects the control that the
embayment has on the distribution of debris
and ephemeral streamflows over the alluvial fan
surface (Viseras et al. 2003).
Arid aeolian systems are particularly sensitive
to changes in precipitation and also to factors
such as temperature, affecting evapotranspira-
tion. The threshold of change for precipitation
lies within the arid-semi-arid transition zone,
where increased vegetation cover becomes more
important. Thus if precipitation increases dune
types may revert from free to vegetationally
impeded (section 5.3.4) and existing dunes may
become stabilized. In addition, aeolian envir-
onments will be susceptible to changes bought
about by changes in wind strength and direction.
Examination of Quaternary records in tropical
