Much of the placer gold recovered in California came from Hydraulic Mines. These operations used pressurized water to wash away great depths of gravel, and the gold was recovered in sluice boxes. For environmental reasons, this practice was ended in the 1880s. Some of the Hydraulic mines were located along river benches, but most were located along the pathways of ancient channels, called tertiary channels, as most flowed long ago during tertiary times. The hydraulic mines cover many tens of thousands of acres in the gold country. Although there were some coarse nuggets recovered from these channels in some areas, most of the gold was fairly fine grained. These same hydraulic mining areas are usually good for prospecting with a metal detector.  My favorite type of hydraulic mine site is what the old timers called a ground sluicing operation. These were smaller scale operations and sometimes did not have the careful clean up that the larger mines did. I have generally done well at these types of sites and have picked up some nice nuggets.
HUNTING THIS SITE WITH THE MXT:
There are some more mineralized areas, but in general, the soil here is warm, but not too hot. I was able to run my MXT at full +3 gain without significant problems or too much noise. There were a few places with alkaline soil (salt in the soil) which made a little noise, but these were small and far between, so they were not really a factor in how I set up the MXT. I used the prospect mode in most areas, but a few places had a significant amount of iron trash, including broken bits of rusty can.  In those places, I used the prospect mode with the trigger forward to disc out the iron. I had the disc set at 3, which is low enough to eliminate most iron, but will allow nearly all nuggets. When in Prospect mode I had the SAT set at 6. The mini 6x4 DD coil was used because of its ability to get small gold.

The MXT will also do well along river where the bedrock is shallow. In places where the river gravels are deep, placer gold tends to work its way downward, and get below the detection limit of the MXT - so look for those areas with exposed bedrock.  These photos show some exposed and shallow bedrock areas along the North Fork of the American River near Colfax and the North Fork of the Yuba River near Sierra City in Northern California.

HUNTING THIS SITE WITH THE MXT:
Streambeds are notorious for hotrocks - the effect varies, but in some places it can be pretty bad. I was able to run my MXT at full +1 gain without significant problems or too much noise here, but high led to too much chatter. Trash is usually also an important a factor in how I set up the MXT at these types of locations. Most have significant trash, so I use the relic  mode in most areas. It is important to remember that lead and heavy iron trash will lay down with gold in crevices in bedrock because both are far heavier than gravel. In those places, you may wish to pan out the contents of the crevice if your detector shows iron or lead in it. Last year I took out more than a penneyweight of small nuggets from a deep crevice with iron trash. The MXT saw the trash, and I guessed there might be gold with it and there was - so you might want to pan the sand and gravel you dig out of those cracks in the bedrock while digging your target, even if the target is not gold.  I had the disc set at 2.5, which is low enough to eliminate most iron, but will allow nearly all nuggets. When in Prospect mode I had the SAT set at 6. The mini 6x4 DD coil was used because of its ability to get small gold.