Gold is heavy and goes to the bottom of the river as deep as it can. it will not be on top of the gravel, normally you will not see gold in the river.
Put three small pieces of lead in your gold pan, if the lead stays with that shiny yellow, when you are done panning off all the sand and gravel when done (the yellow) after panning off the heavier back sand is gold.
Try panning, with a few pieces of lead.
http://goldrefiningforum.com/phpBB3/search.php?keywords=Try+panning+with+a+few+pieces+of+lead&terms=all&author=Butcher&sc=1&sf=all&sr=posts&sk=t&sd=d&st=0&ch=300&t=0&submit=Search
Nitric acid will dissolve pyrite, not gold, but is really not needed once you know what gold is.
Break open the cracks in that bedrock dig it out and pan that.
Dig down to the bottom of the pile of gravel to bedrock (clean cracks).
Check bedrock behind big boulders, or bedrock pockets.
Gold settles on inside bends (under thick layers of sand and gravel), outside of river bend gets washed out.
Check grass roots on bank (for very fine gold).
gold and black sand like each other (where you find one the other is normally close by).
Look for conglomerate (packed sand and gravel {cement} just above bedrock.
On moutain tops or sides look for round rocks (ancient river beds).
Like anything else you wish to learn, study about it, today with library's of information at our finger tips a simple search is the beast place to start to find gold and identify it, either on the forum or google...