Hi amethystguy,
Good question to pose. Hope my response is useful as well as this section of the forum you found.
I'm not a lawyer, but as I understand it, The US Corps of Engineers regulates the water and delegates overseeing/enforcement to the states....just the water.
As for the property ownership/rights under that water.......that's a whole different story. Yes the center of creek can be used as property boundry but not always, could be "center of creek bed" or may not use creek but cover it completely with large tracts. Who ever owns that property or has valid claim to ground under water in the creek, river, under roads or powerlines etc owns that ground and minerals within.
Having the owners permission is a definate thing to obtain for prospecting in those cases
.
National lands as well for waterways......US Corps of Engineers for the water authority but does not make ground under water open......there can be and often are claims for placer aluvial deposits in these type areas.
From one state to another there could be several small differences such as fencing off across creeks.........see alot of it in Colorado but not allowed in other states. Still though, the basic above should be consistent I think.
State forest also may differ since they are owned by that state and not national lands concerning mineral prospecting. States can implement their own regs/rules for those.
I could be wrong though......
sometimes it's very confusing!
CP