I saw a minnow swimming in a terrace holding area, about 1 acre in size, 2-3 ft. deep at most. This holding area had been dry 6 weeks previously, and will hold water for about 30 days after a large rain in central Tx.
There is nothing uphill of this area; there is a creek about 1,500 ft. away and 30-40 ft. downhill. Either the minnow swam upcreek after a rain, or a bird brought him. There just seems to be no other explanation. Sure someone could have stopped and droppedit in the ditch, but that seems highly unlikely.
On the other hand, I've caught boys trespass fishing at pond in Louisiana, and found in talking to them that they had "stocked" crappie in the pond (thanks), so that does happen.