I know this is a broad question. We have a place in Concho County, on the Concho River that has a large plowed field on the river. After a rain we always find arrowheads and tools on these rocky humps in the field. The rocky part looks like burned rock and mussel shells but its really spread out and not real concentrated. I want to try and dig but dont really know where, how deep, ideas would be greatly appreceated. Thanks

Sounds like you've got some middens in that there field; which makes good sense if its well out of flood plain. Find the very edge of one of these humps you speak of and go straight down in a decent sized area (5' x 5' or larger) until you see no sign of flint, muscle shell, snail, or burned rock... and then I would go a foot underneath that just to be sure I was at the bottom (often times "sterile layers" from floods left a layer of sediment that contains near no sign of occupation). The contents of your test hole will tell you alot.

Sounds for sure like a midden is close to me; definately gotta start a test hole - there is no other way to know.

If you don't turn up the evidence you seek then test the other "humps". If still nothing then perhaps the treeline farthest from the river, nearest the field is a good place to look... perhaps over the years the river has shifted.