In general, the risk you are willing to take the the possible return on your investments.

most sediment washes or falls into the river as a result of mass movement and runoff. other sediment eroded from the bottom or sides of the river.

they it depends on if one believes about radioactivity, they make a geiger counter beep but maybe it was going to beep anyway electronics do wierd things all the time or it was programmed to beep near certain metals.

answer seismograph;