Drill hole in river bedrock

Can anyone think of plausible reasons why I would a perfect drill in the river bedrock and above the normal water line?  The nearest road crossing is a low water bridge .5 miles up river.  The next crossing is nearly two miles downriver.  The drill hole is about 1" - 1 1/2" in diameter.  When I get back there this summer I will get pictures.  I should be going back in June.  Thanks.  If anyone else has river bedrock drill hole pictures, by all means post away. 

Here is a couple from the Domiguez Canyon with the drills left in the last holes.
Our belief is that they were used as a ferry anchor.

I found this about a quarter mile up stream from my indian cheif carvings.  There is an old mill (1700s) foundation not too far away as well, so it may be related to that.

The water is not deep enough for a boat to be anchored with it, so I really have no idea why its there.

could the water have been deeper in those days?

Along the Colorodo river I have seen a lot of them  . They were used as points to attach the winch lines of  ferry boats in years past. Depending on the snow melt / storm water flow / rapids  and laden weight of the boat , some times a winch line was used to help the trip up stream.

we have some drill holes in rock at a local swimming hole.  there was a t one time a grain mill there. all that is left is some holes in the granite.  the water was never deep enough for any type of boat. 

Along the Colorodo river I have seen a lot of them  . They were used as points to attach the winch lines of  ferry boats in years past. Depending on the snow melt / storm water flow / rapids  and laden weight of the boat , some times a winch line was used to help the trip up stream.

I like that.  As dsty mentioned once, it could have been for modern dredgers as an anchor point and could also fit extractor's view.  Quite possibly it could have been used to wench small barges upstream/downstream which means that there should be other holes.  I have at least four days every year when I go there.  I will definitely be searching that whole stretch for more clues.  Thanks for giving me the 'bug' fellas.  Nothing could be too complicated at this point.  The more scenarios or suggestions I have, the more creative I can be in my searches of unobscure locations.

The ones I pictured above  are on the Gunnison River.
If you attach a line to the bow of a boat just back from the prow.
at a gentle curve of the river depending on which side of the boat the line is attached ...
The current will do the work and send the boat from oneside to the other.
Swap sides with the line and the boat will come back.

Simple engineering.
The Spanish used it.

Your right on DSTY. Sometimes I put to much into the signs and make it too hard to figure out. Thanks to you, Old dog and dign I'm beginning to think on the right path. Thannks guys. RH2

Your right on DSTY. Sometimes I put to much into the signs and make it too hard to figure out. Thanks to you, Old dog and dign I'm beginning to think on the right path. Thannks guys. RH2

I agree with and appreciate everyone's input.  New things are often complicated to those of us who lack knowledge and experience of new ventures.

OldDog:  I really wish I had known of such things when I discovered that three rock formation.  The one I disturbed.  However, with the help of GE and a lot of refreshing about the area, I have found three points of contact which closely resemble a reversal of the picture I took.  I remembered that some signs are reversed.  When I mirrored the image and placed it over the location, it seemed to make a nice geometric match.  The two natural landmarks, represented by the small stones (after reversal), also pointed a line to the location where the formation was originally found, nearly 7,000 feet away.  Of course, with such horrible resolution, in low traffic areas, exact distances with only happen with a wheel or GPS.  Maybe all hope is not lost of finding anything.  Now it may be a bit more difficult but, I don't believe impossible.