[Talk-us] How are US county boundaries legally defined?

[Jake]

I've been mapping a small section of National Forest, which straddles two
counties; Boone and Callaway.

On every map I can find - Boone Countys GIS dept., census.gov, US Forest
Service - the county border strictly follows a river, Cedar Creek. However,
on OSM, the boundary is shaped exactly like the river, but is shifted about
a quarter mile north-east of it. Here's a small section to show what I mean:

https://www.openstreetmap.org/#map=14/38.8117/-92.1427

Now, I'm pretty sure this is a mistake - rivers move, but they don't shift
in perfectly synchronized 40-mile segments like this.

I'd like to find out how that boundary is actually legally defined, but my
google-fu is not strong enough, it seems.

US mappers - do any of you know what government body is the keeper of truth
for Missouri county boundaries?

Best,
Jacob
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