Geoscience Reference
In-Depth Information
review, however, is that when all dated lake levels are plotted together as histograms
showing the number of dates relating to high, low or intermediate lake levels at suc-
cessive stages in time, then a majority of lakes in presently arid and semi-arid North
America had high or intermediate levels during late glacial times. The hypothesis of
glacial
=
pluvial was thus supported for this region on the evidence available at that
time.
The unresolved question hinged on the relative importance of all of the factors
controlling the water balance in particular lake basins. Earlier studies had begun to
tackle this problem. For example, water balance studies by Leopold (
1951
) and by
Antevs (
1954
) based on late Pleistocene snow-line fluctuations in the mountains of
New Mexico suggested that pluvial Lake Estancia was high at that time because
of a combination of decreased temperature, decreased evaporation and increased
precipitation. Reeves (
1965
) investigated the pluvial lakes in the Llano Estacado
of west Texas and concluded that Pleistocene precipitation was little different from
that of today in this region. He concluded that fluctuations in run-off modulated by
temperature fluctuations were the primary cause of the high pluvial lake levels in west
Texas.
Pleistocene Lake Bonneville is one of the most studied pluvial desert lakes in
the world. First investigated in detail by G.K. Gilbert (
1890
), who identified three
well-defined high lake shorelines and evidence of a major overflow channel, as well
as deformation of strandlines as a result of isostatic readjustment, at its maximum
Lake Bonneville was more than 330 m deep and occupied an area of 51,640 km
2
,
with a total water volume approaching 7,500 km
3
. Gilbert named the high shorelines
he identified Bonneville (around 1,565 m elevation), Provo (around 1,470 m) and
Stansbury (around 1,350 m). Because of isostatic rebound after loss of lake water,
these are best regarded as relative elevations, given that the same strandline will appear
at different elevations across the basin. The Provo shoreline started to form after the
lake had stabilised at this lower level following the breaching of the alluvial divide,
described later in this section.
O'Connor (
1993
) studied the hydrology of the overflow of Lake Bonneville that
took place near Red Rock Pass in Idaho some 14,500 years ago and sought to estimate
peak discharge in ten separate reaches along the flood route. Using geomorphic
evidence, he estimated that peak discharge amounted to 1.0
10
6
m
3
sec
−
1
at the
Lake Bonneville outlet near the Red Rock Pass. Estimated values of stream power
below the outlet ranged from 10
1
watts m
−
2
in ponded reaches to 10
5
watts m
−
2
in
constricted reaches. Stream power (discussed in
Chapter 10
) was defined by Bagnold
(
1966
) as the rate of energy loss per unit length of stream. Both stream power and
sediment transport rate are roughly proportional to streamvelocity cubed. Cobbles and
boulders carried by the flood ranged from 10 cm to more than 10 m - testimony to the
extraordinary magnitude of the Bonneville flood. Once the lake had overflowed and
lowered the level of the sill forming its outlet, it was, of course, never able to exceed
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