The mississippi river is full of sandbars and many a boat became stranded. For this reason, wise captains assigned someone to constantly measure the depth of the river when they knew (or suspected) that they were in shallow water.

They would hold a rope over the side of boat with a weight at the bottom and constantly watch the depth. When the critical depth marked on the rope (2 fathoms) was reached, the person would yell out as loud as he could "Mark Twain!!" Mark as in "we hit the critical mark" and Twain is old English for "two."