Alluvial deposits are the broad term applied to placers formed by the mechanical action of moving water of streams, lakes, or oceans. One can tell how close the alluvial deposits is to the source vein by the appearance of the gold. As gold is trasported, the flakes become rounded and flat. Therfore, the more angular the gold flakes, the closer the deposit is to the original source. The source, however, may be less concentrated in gold than the actual placer deposit.
Most recreational mining involves alluvial deposits along stream beds.
Gold, being heavy, will settle whenever there is an area of slow moving water. Areas that typical concentrate gold are the inside bend of a curve, behind large boulders, etc. For a thorough and easily understood description of where placers form in stream beds, we highly recommend Gold Mining in the 1990's by Dave McCracken. Click on the title to order the book from Barnes and Noble. When first starting, crevice mining offers an easy and inexpensive way to get started.