Maybe someone can help me put a few things together.  On a river where 15' is deep, all my catch is between 6 and 8'.  But I am not finding them deeper than that either.  Spawn is just about done maybe a couple stragglers behind.  We have had some unseasonably cool weather and a decent amount of rain that cooled the water.  Then temps that climbed for a bit and warmed the shallows up.  Then cooled again followed by warm and is now climbing back up.  These were sustained changes for decent time periods.  I have tried fishing shallow (less than 6') in every possible situation.  My thery is that the bass have found a threshold at these depth because the water temps are not fluctuating as quickly.  These depths are shallow enough for some spawn and probably stage post spawn fish fattening back up to retreat to their summer hide holes.  Does this some right or am I missing something?  Helpful to know you know something and not just thinking ya know something.  (hence my search for confirmation)

You've found their comfort zone!

I'd stick with it & wear em out!

Post spawn bass may suspend above an off-shore thermocline. It's either too cold for comfort below that depth or else the forage schools are at or above the depth where the bass are suspending. If you measured temperatures below your boat you'ld probably find a sharp gradient just below the fish.

I fish the Mississippi river and the current keeps the temps and oxygen mixed up throughout the water column . I cant recall ever catching bass deeper than what you reported . 

In the rivers I fish, 15 feet would be as deep as it gets, but I get all my fish no more than 4 feet deep, usually less. Like what was posted before, the current keeps the water temp more constant from shallow to deep.  I usually associate deep water in rivers to inactive fish so I spend my time chasing the more active, shallow bass.

Thank you fellas.  This river has very little current.  Its there but its hardly moving.  If they open up a lock and the river moves a little the fishing is awesome!  Shallow bass all day long.