is it true about the meaning of mark twain?

it sounds like its bs. anybody know? how can a name be used to tell how deep the river is? unless it was a unit of measure...like "well, the river is two mark twains right here, but only one mark twain over there."

edit- just read on wikipedia (not stating that is solid evidence of course) that a captain isaiah sellers used to write notes while sailing the river about its condition and sign them mark twain.