After reading the BUS&SS section entitled "How Boundaries Are
Established and Changed" (BUS&SS, p. 3), I have a theory that solves
the conflict between the witness post lying 9 chains west of the Bois
de Sioux and the USGS Boisberg topographic map. The following
paragraph is excerpted from BUS&SS:

Where running streams are the boundaries between States, ... when the
bed and channel are changed by the natural and gradual processes known
as erosion and accretion, the boundary follows the varying course of
the stream [see 265 U.S. 499]; while if the stream from any cause,
natural or artificial, suddenly leaves its old bed and forms a new
one, by the process known as avulsion, the resulting change of channel
works no change of boundary, which remains in the middle of the old
channel [or on one bank if so fixed by statute], although no water may
be flowing in it [246 U.S. 173].

My theory is that, at the time the witness monument was set, the Bois
de Sioux (and boundary) ran in a channel 9 chains to the east. It
consequently migrated west to the position shown on the topo map.
Since this was a gradual process of erosion and accretion the boundary
moved west as well. Some time later the Bois de Sioux channel was
leveed and straightened to it current position. This would have been
an artificial process of avulsion and, therefore, would not have
changed the boundary. Thus the migrated-to boundary was frozen in the
position indicated on the topo map.

I have no evidence beyond that presented here, but this theory should
be verifiable or refutable if I can obtain some additional
information, which I will attempt to do now that I know what I'm
looking for.

Regards,
BJB