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Figure 1. Location maps of study area. (A) Major tributaries of the Mississippi River system in relation to the maximum extent of all Quaternary glacia-
tions, shown in white. The unglaciated Driftless Area (DA) shown in upper Midwest. (B) Location of the upper Mississippi River and Wisconsin River in
relation to the maximum extent of MIS 2 glaciation, shown in white. Area of Figure 1C shown by red box. (C) LiDAR-derived hillshade image of the lower
Wisconsin River valley and confluence with the Mississippi River. The three remnant segments of the Bridgeport strath are located within the white
boxes, identifying areas of detailed maps in Figure 2. The white circle immediately south of the confluence of the Wisconsin and Mississippi Rivers
indicates the location of the town of Wyalusing, Wisconsin, USA. Geomorphological features of the lower Wisconsin River valley that indicate drainage
reorganization has occurred: barbed tributaries of the lower Wisconsin River (pale blue arrows); the curved inner valley wall at the confluence of the
Wisconsin and Mississippi Rivers (solid orange line) more similar to the inside of a bend on a single river (example identified by dashed orange lines);
narrow reach of the Mississippi River immediately downstream its confluence with the Wisconsin River (yellow bracketing arrows).

It is within the context of stream piracy 1922). Buried bedrock valleys, modern 1918), an observation that has been recog-
and routing of freshwater off the North streams under-fit to the bedrock channels nized since the 1820s (Martin, 1932). It is
American mid-continent that we investi- in which they flow, and river courses bounded on the east by MIS 2 glacial
gated the lower Wisconsin River valley in aligned to former ice margin positions are deposits, and on the north, west, and south
the Driftless Area of southwestern common features. Furthermore, late by older glacial sediment. Regionally, the
Wisconsin (Figs. 1A and 1B). As an iso- Quaternary glaciations drove sequences of Paleozoic sedimentary bedrock is heavily
lated area of unglaciated terrain north of aggradation and incision, producing mul- dissected by fluvial incision (Trotta and
the overall maximum extent of Quaternary tiple cut-and-fill terraces along the upper Cotter, 1973) that is expressed in the hilly
glaciations in North America, the Driftless Mississippi and lower Wisconsin Rivers surface morphology because of the lack of
Area provides a much longer temporal and their tributaries; several outwash ter- Quaternary glacial deposits in the
window to landscape and drainage basin races are graded to a higher elevation than Driftless Area. While a traditional expla-
evolution than in the surrounding glaciated the modern floodplain surface (Flock, nation for the particularly deep incision of
regions. This allows the opportunity to 1983; Knox, 1996). In the North American the upper Mississippi and lower
reevaluate the late Cenozoic evolution of mid-continent, however, the lower Wisconsin Rivers and their tributaries is
the upper Mississippi River basin, and to Wisconsin River is atypical among major simply surface expression of long-term
assess the impact of diversion of freshwa- rivers for containing prominent remnants process, a compelling argument will be
ter runoff from the North American mid- of a strath (bedrock) terrace. This surface, made here that the lack of glacial cover in
continent away from the North Atlantic known as the Bridgeport terrace, is found the Driftless Area affords a window to
Ocean and toward the Gulf of Mexico. at a higher elevation than adjacent deposi- view late Cenozoic drainage integration of
tional terraces along the Wisconsin River. the upper Mississippi River basin.
STUDY AREA AND BACKGROUND Three isolated remnants of the strath
occur within 60 km of the confluence of Within the lower Wisconsin River val-
The upper Mississippi watershed is a the Wisconsin and Mississippi Rivers (Figs. ley, Knox and Attig (1988) identified a
major sub-basin of the greater Mississippi 1C and 2). moraine and glacial outwash consistent
River system that has been significantly with a glacial advance from the west to a
impacted by Quaternary glaciations. The The lower Wisconsin River flows west few kilometers east of the confluence of
upper Mississippi and Wisconsin Rivers from the Baraboo Hills in south-central the modern Wisconsin and Mississippi
and their major tributaries (Fig. 1) all Wisconsin through the Driftless Area to Rivers (Fig. 2A). The outwash, preserved
cross the MIS 2 glacial margin and exhibit its confluence with the Mississippi River. on the Bridgeport strath, contains east-
the effects of multiple Quaternary glacia- This region of southwestern Wisconsin ward-dipping foreset bedding, indicating
tions on their geomorphology, planform, was apparently never glaciated during the that water flow at the time of deposition
and course (Warren, 1884; MacClintock, Quaternary (Chamberlain, 1883; Alden, was in the opposite direction as flow of

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