I haven't a clue where they came from, but we now have grass shrimp. I have done a couple searches and a little reading and actually saw them in some flooded grass Sun. They were first discovered a couple months ago by my son (davatsa) while raking primrose. This years BGs are almost 3/4" and are having a feast on the flooded shorelines. We have a LMB that lives in 8" of water, 2' from a feeder. It waits for someone to walk by and spook the minnows and nails them 3-4' from the shore. 2-3x a day it performs!
2 of 3 ponds now overflowing, and have been for several days!