Another fine TR. And I agree that waking up with a 60 year-old woman is a fine experience!
The construction of ditches to provide water for hydraulic mining or just conventional sluicing is a fun topic to study. The water was often provided by a "ditch company" which was capitalized and operated to produce the water and deliver it to miners. The pricing was by the "miner's inch", resulting in a series of calculations which brings the advantages of the metric system into full view. Entire crews of ditch company workers "rode herd" on the ditches to keep them repaired and maintained. If the notched boulder provided a good view of a long segment of the ditch, it's quite likely it was created as an observation post for ditch workers, who were also responsible for prevention of misappropriation of the ditch company's saleable product--the old saying being "whiskey's for drinkin'; water's for fightin'".
There were likely many more ditch company owners enriched than miners. Sort of like the suppliers of hardware, tools, and other supplies.
Foy