Here is an image of what we found lying at the bottom of the river between the old bridge piers. The center of the image is the sonar head location at hole # 3.
The object near the sonar instrument underwater is about 6.3 m (20 feet) by 1.2 m (4 feet) and as you can see is almost directly between the two piers. The top pier is the pier closest to the South shore and is located approximately 25 meters from it. The center of the scanned area in the following image is 60 meters from the South shore.
The following picture puts everything in perspective. I drew a red line approximately where we have located the anomaly. This picture was taken in the late 1800, more than 30 years before the dam construction. The water level is clearly very low and yet the second, third and fourth spans have water running underneath them.
I'm busy working on my blog posts. Watch this space!