Fire rock & flint flakes is a solid camp indicator, finding good artifacts is only a question of moving dirt.
2 thoughts....Be VERY SURE to go deep enough, well below the obvious black soil.. #2,,,make a 2X4 frame with 1/2 inch wire mesh screen...Could be a bird point camp with very small points that are hard to see unless screened.
Are you finding flint? Or flint chips that would show signs of flint being worked into actual tools/points? 30 yards from an old river bed may be to close for a midden unless its on higher ground.