When you consider just how flat the area is in general, why did nobody wonder where the lakes came from? Hardly glacial.
Many will also notice that gravel pits and quarries are not too hard to find in the surrounding areas, so I can't help but wonder whether there are some flooded ones too in the waterways area.
If it is true that the water levels rose and then receeded once more? Then one can imagine that the actual Broads or lakes themselves may have been hidden and formed much more part of the flowing rivers.
I found the reference to sand deposits most interesting, as everywhere I look, from in the water to out of it, is sand, and quite often water rounded gravel too.