THis is a great questions and great answers above, I cant add too much that hasn't been said accept i will say there are some areas which gold will drop out favorably and if you understand that by monitoring the flow over different seasons you will rarely dig in an area without gold.
In order for pay layers to form you need good deposition, deposition comes from a constant feed of alluvial gravels, understanding how gold is deposited into the system in the first place is significant i.e. is it eroding down the gullies from bedrock and is super fine working its way into the creeks? this fine powdery often granite pounded gold will tend to collect in the high flood deposits due to being upheld in solution by silt, it wont be on the bottom that's for sure.
**This is just under a gram panned from the upper inside bend of a creek held up by honeysuckles, deposit was fine sand note how the gold floats..... yes gold floats especially when contaminated after a flood
If you are in an area that is fed by deep leads you will fine that there is new heavy nuggety and often busted up reefy gold deposited right up out of the basalt columns, this type of gold will carry on the bedrock or clay/decomposing bedrock layer and it gets there rather quickly.
****heres an example of some gold got between two flat slate riffles 10 meters apart from the bottom*** indicator was crunchy gold all the way down
If you are in an area with course and rounded quartz and the shape of the creek changes with the season then you know that quartz will be busted open and more gold will be deposited from the quartz periodically.
Back to the original question, do i dig to the bottom?
Firstly look at the geology do you see natural riffles? bi rock bars? is the bottom soft or rough?
Smooth bottoms do not make good gold catchers, regardless of how good a deposit is it is much easier for gold to keep on moving even with a clay layer on a smooth bottom then it is settle during times of significant turbulence which is when it is most active.
Concentrating on rough bottoms can improve your chanced exponentially, it also makes cleaning the bottom all that much more difficult which is why we love it cos on occasion you can pull a nice picker that has been missed by every other prospector.
Thats not to say that you wont find a good deposit on a smooth surface either.... when is a smooth surface a good prospect?
a. when you have a thin section of creek which flushes clean in high volume period followed by a outward trend
excuse the crappy picture
b. When a smooth surface is followed by a large perpendicular natural riffle which i believe to be the best prospects. Gold finds it very hard to move up and over these shelves and working just one foot before these can prove very productive. These can be upward riffles or DROP riffles.... drop riffles like the one below are amazing!!! and when they cover the full width of a reek running downstream you can bet the downstream side will be in most cases the best place to start..... go to the bottom if you find a drop riffle especially after a long section of fast moving water.... hope this helps