Sandman, how high above normal river level is the surface of your pond? Not counting rain, does your pond rise and fall with the river's level?

If your pond's water is on the same level as the river, digging it out would be worthwhile.

If your pond's water is higher than the river's level, you may run into the old river bed, and cause your pond to drain.

Rivers always meander over the centuries, and it's almost certain that you will hit sand and gravel if you dig. If the old beds are within 1/4 mile or so from the river, the old beds almost always communicate with river. That means that your pond will rise and fall with the river, if indeed it isn't already doing so. If it is communicating now, digging it deeper would work, and your water would always stay fresh and cool.
In that case, fertilizing would not work well, but feeding would help.

Shrimp