My first guess would be it came from all the birds you have there. Birds transfer more junk on there feathers and feet it's amazing. I don't know why anyone would plant such a weed? Raking would be one heck of a chore in that size pond. I have a one acre pond and it gets old real quick I couldn't imaging doing 15 acres.

The only difference between a rut and a Grave is the depth. So get up get out of that rut and get moving!! Time to work!!