Here is a thread about my adventure of cleaning out my son's old pond that was about 50 years old. If you look at the pictures at the beginning or the before and after closer to the end of the thread, you can see that old pond had silted in and the dam had been breached to the point it would only hold very shallow water part of the time.
Reclaiming a 50 year old pond Yours might be something similar. Just many years of silt and neglect.
Before and after pictures about half way down on page 2
