When you are sampling for gold in a streambed, you should nearly always be looking for hard-packed material. “Hard-pack” is created at the bottom of waterways during major floods and storms. The reason that hard-pack is important to a prospector is because gold nearly always concentrates at the bottom of hard-packed layers. Therefore, it is nearly always important for a prospector to target his or her sampling efforts to reach the bottom of hard-packed streambeds.

Gold is about six times heavier, by volume, than the average weight of the sand, silt, and rocks that make up an average streambed. Because of this disparity in weight, when streambed material is being washed downriver during a major flood, most of the gold will quickly work its way down to the bottom of the streambed material. Because the gold is so much heavier, it will work its way down along the river-channel more slowly than the other streambed materials. During major storms, most of the gold moving in a waterway will be washed down across the surface of hard-packed streambed that is not being moved by the storm. At some point during the storm, gold becomes trapped out of the turbulent flow by dropping into cracks and holes. Streambeds form later in the storm, when the water-turbulence tapers off enough to allow the rocks, gravel, sand and silt to drop out of the flow and form a layer along the bottom (over top of the gold).

Streambed material that lies on top of the gold will nearly always be hard-packed. Why? Well, if there is enough force and turbulence to move substantial amounts of gold in the waterway, then there is also enough force to create a naturally-formed streambed on top of the gold as the same storm and flooding dies down.