Arikaree River - lowest point in Colorado
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USGS tells me that, when they created their list of low points (in the 1980s, I think), they used the smaller-scale 1:250K map series, not the largest-scale 1:24K maps. Perhaps it was because at the time there was not universal coverage at the 1:24K scale. Or maybe they thought it would be an unnecessary waste of time to get that detailed by looking at so many maps! Anyway, when I look at the 1:250K quads in question, I tend to believe their explanation. You can decide for yourself below.
I've posted the portions of each map that show where the three rivers flow outside Colorado's eastern border. Each of these maps have 100-foot contour intervals, with 50-foot supplementary (dashed) contours. That, I believe, is the key: the stated contour interval is uniform over the entirety of any given map, but "supplementary" contours are included only at the discretion of the cartographer.
This first map below is from the "Sterling" 1:250K quad, showing where the North Fork of the Republican flows into Nebraska:
The thicker brown line is the 3500' contour. So, using this map, all one can say for certain is that the elevation of this river at the state line is "less than 3400 feet".
The point where the Arikaree River flows into Kansas is just to the south, on the adjacent quad, "Limon" (below):
Note that here, too, all you can be sure of is that the Arikaree is less than 3400 feet.
Now, check out this portion of the "Lamar" quad that shows where the Arkansas River flows out of the state (below):
Note that the cartography includes "supplemental" (dashed) contour lines for both 3450' and 3350'. This allows one to be certain that the Arkansas River is under 3350' when it exits the state.
So, based on this scale map series, I can see how someone might come to the conclusion that the Arkansas River has the lowest elevation at the state line.
But now we know that it pays to look at a more detailed map. Perhaps your state's low point is incorrect, too?...