Be very careful with the hike upstream looking for sandy gravel. I own a part of a creek that can be followed upstream from a road. It gets into places, on my own land, where it looks pretty darned wild and might be the place that WD is talking about. At that point it is just a few hundred yards from where my farm fields and my neighbors farm fields run off into it. It looks pristine and wild down in the woods below the fields but I know that it has the residue from chemical run off since long before I owned the place. If you kept following and walked uphill to check things out, you would find my unfertilized tree plantation, I converted an unproductive field to forestry uses. That would convince you even more that the creek was safe, but the chemicals from my upstream neighbor would still be there along with who knows what from the person that owned my property before me.