I think this will be because the dog harness which was found on the river bank was wet. This would be because the owner had gone down to the edge to get it, slipped into the water accidentally then thrown the harness onto the bank so as to be able to use both hands to get out of the River.
Very sadly, IMO, this attempt to get out was not successful.
It's awful but IMO she is in the deep hole part of the River somewhere. There is a fair chance that sonar is not the answer for this. I don't care how much the computer kit cost it is how well it works in reality that matters rather than the dollar value.
(Edit for typo)