The M.E. tested diatom levels in the bodies and it turns out the unsub drowns his victims at different locations every time, not just where he abducts them or where the bodies are later discovered.
n. [ Written also levelling. ] 1. The act or operation of making level. [ 1913 Webster ]
2. (Surveying) The art or operation of using a leveling instrument for finding a horizontal line, for ascertaining the differences of level between different points of the earth's surface included in a survey, for establishing grades, etc., as in finding the descent of a river, or locating a line of railroad. [ 1913 Webster ]
Leveling instrument. See Surveyor's level, under Level, n. -- Leveling staff, a graduated rod or staff used in connection with a leveling instrument for measuring differences of level between points. [ 1913 Webster ]