There might be a reason you started there don't know,but plan (b) could be to check a different aera to start.Unless this is the only access spot to start for loading reason or other???
Set a pad out just to stand on to probe(measure) and get an idea how dig the silt is down to solid dirt.There is usual soft spots on the floor of the ponds.Like where the old ditch flowed into the pond .It is always softer because lot of times when building the pond they didn't get dirt there to build with.
Other places they didn't go as deep to start with .Like closer dirt,so they dug a hole there ,but on the other side they didn't need as much dirt,so might only be a few feet down to solid bottom there.
The elevation of the bottom can vary a lot.The ones i have cleaned has not been as large an area.Here small area ,but they are deep.Like the ones you have showned are big area and look to be shallow compared to here.
Here which is just for livestock use.We usually start at the dam edge,bench the front slope out.Set the excavator there.Dig a start area by throwin the silt over the back side of the dam.Then take the dozer,keep pushing the silt to the excavator.Start working our self across the front of the dam left to right or what ever works.
As we get down to the solid bottom with the dozer while the excavtor is resetting and starting new dig.The dozer is refacing the front slope of the dam(pond) when your done it looks like new pond except for the slop on the back side.Which is left to dry for a few years then come back and reshape the backslope.
So will you be cleaning the whole area or just an area like for a boat ram or etc.?Good save and good luck.