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initiated stream piracy events that reorga-
nized those river systems to become the
modern Ohio River that drains to the Gulf
of Mexico (Fig. 4). An isotopic signal for
this reversal may be preserved in Gulf of
Mexico sediments (e.g., Joyce et al., 1993;
Shakun et al., 2016), although the clarity
of such a signal would be a function of
whether all drainages in the Midwest and
Appalachians were rerouted in a short
period of time or over multiple glaciations. flux are based on modern morphologies
As outlined here, the ancestral
Wyalusing River was also a tributary to
the St. Lawrence River system prior to
Quaternary glaciations. As such, a com-
mon mechanism for the reorganization of
the Ohio and upper Mississippi Rivers
during the Quaternary is logical and
appealing. The early to middle Quaternary
glaciations that blocked the lower St.
Lawrence drainage and caused the reorga-
nization of the modern Ohio River neces-
sarily would have also blocked the ances-
tral Wyalusing River in the Midwest. This
provides a single causative agent for reor-
ganization of drainage systems across the
eastern and Midwestern United States.
Farther to the west, evidence exists that
the area currently drained by the Missouri
River was modified such that the modern
Missouri River closely follows the MIS 2
margin, though it may previously have
contributed additional drainage area and
runoff to the Gulf of St. Lawrence–
directed system.
Hemispheric Implications
The area of the combined ancestral
Pittsburgh, Teays, and Wyalusing River
basins is significant, representing at least
~420,000 km2 of the modern Mississippi
River basin that has been pirated from the
pre-Quaternary St. Lawrence River basin.
Because these areas are located in the rel-
atively humid portion of the larger
Mississippi basin, they represent a dispro-
portionately large amount of the
Mississippi River’s discharge. Analysis of
modern gage records indicates that these
pirated basins represent ~14% of the area
of the Mississippi River basin yet contrib-
ute nearly one quarter of the mean annual
discharge of the Mississippi River (Carson
et al., 2014), roughly equivalent to 150
km3/year of water (a permanent diversion
of nearly 5,000 m3/s, based on modern
hydrology). While this amount of fresh­
water is small relative to late Quaternary
outburst floods that temporarily disrupted
North Atlantic thermohaline circulation
(e.g., Teller, 1990; Licciardi et al., 1999; ally exposed unweathered bedrock across
Clark et al., 2001), piracy of these basins
in the midcontinent and flux of that water
away from the Gulf of St. Lawrence and
toward the Gulf of Mexico represents a
permanent step-function decrease in
freshwater delivery to the North Atlantic
from a non-climatic source. These estima-
tions of drainage area shift and discharge
and flow regimes; the redirection of gla-
cial meltwater toward the Gulf of Mexico
following reorganization would only serve
to further increase the significance of
drainage reorganization on freshwater
delivery to the Gulf of Mexico (Wickert et
al., 2013; Wickert, 2016).
It has long been understood that the
delivery of freshwater, and particularly
fresh meltwater during glaciations, exerts
a significant control on North Atlantic
thermohaline circulation. Multiple studies
(Broecker et al., 1989; Condron and
Winsor, 2012; Ivanovic et al., 2017) have
shown that a large pulse of meltwater was
the mechanism that initiated the Younger
Dryas by reducing Atlantic Meridional
Overturning Circulation (AMOC), which
led to cooler air and surface temperatures
and increased ice cover. However, previ-
ous studies have focused on the effects of
a large, discrete meltwater pulse derived
from the demise of a North American ice
sheet. The data and interpretations pre-
sented herein raise the question of the
ability of a much smaller, though perma-
nent, flux in continental runoff caused by
drainage reorganization to impact North
Atlantic thermohaline circulation.
For example, the middle Pleistocene is
noted for a step-function shift in the peri- that incision of the lower Wisconsin River
odicity of glacial maxima from a predomi-
nantly 41-k.y. cycle to a 100-k.y. cycle
(Shackleton and Opdyke, 1976). This
Middle Pleistocene Transition (MPT) took
place between 1250 and 700 ka. Lacking a
stochastic cause for this shift in glacial
periodicity derived directly from orbital
forcing, a deterministic mechanism is
required. Numerous mechanisms have
been proposed to explain the occurrence
of the MPT, including physical processes
associated with calving and meltwater
discharge feedback (DeBlonde and Peltier,
1991); long-term deepwater cooling
(Tziperman and Gildor, 2003); or the pro-
gressive erosion of regolith from the North
American continent during successive
www.geosociety.org/gsatoday
early Quaternary glaciations that eventu-
the craton (Clark and Pollard, 1998; Clark
et al., 2006). The budget of freshwater
delivery to the North Atlantic Ocean is
one of the major determinants of the
strength of North Atlantic thermohaline
circulation (Clark et al., 2002); the strength
and structure of North Atlantic thermoha-
line circulation, in turn, plays a critical role
in driving global heat transfer and climatic
fluctuations. For example, coupled thermo-
haline circulation and energy balance cli-
mate models (e.g., Sakai and Peltier, 1997)
demonstrate climate sensitivity to freshwa-
ter runoff from ice sheets. While this and
similar studies explicitly investigate glacial
versus non-glacial conditions during the
late Pleistocene, a future avenue of investi-
gation would be to assess the effects of a
permanent step-function flux of freshwater
away from the North Atlantic and toward
the Gulf of Mexico. This may provide
insight into the effect that reorganization
of continental drainage systems may have
imparted on the thermohaline circulation
in the North Atlantic Ocean, thus provid-
ing an alternate explanation, or contribut-
ing factor, for the change in periodicity of
glaciations associated with the MPT.
CONCLUSIONS
Coring to resolve the elevation of the
bedrock surface of the Bridgeport strath
along the lower Wisconsin River valley
indicates that the strath surface dips to the
east at an estimated slope of 0.15 m/km, as
opposed to the westward dip of the bed-
rock floor of the valley, the modern flood-
plain surface, and all late Quaternary dep-
ositional outwash terraces. The direct
conclusion drawn from the coring data is
valley was achieved by an eastward-flow-
ing river during the late Cenozoic, rather
than by the westward-flowing modern
Wisconsin River. Numerous geomorphic
features along the lower Wisconsin River
valley support the interpretation of a
reversal of flow and reorganization of
drainage patterns at some point in the
past. Investigation of a buried bedrock
valley in east-central Wisconsin confirms
that this feature represents the down-
stream continuation of the river referred to
herein as the Wyalusing River. Having
been traced into the Lake Michigan basin,
we conclude that this river evolved as part
of the headwaters of the St. Lawrence
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