An alluvial deposit is an ancient river-washed rock and gravel bar that may be thousands of feet from the nearest stream, creek, or river. Alluvial (or bench) deposits contain untapped potential for finding gold because such areas have never been worked before.
Alluvial placers are usually remnants of deposits formed during an earlier stage of stream development and left behind as the stream cuts downward. The abandoned segments, particularly those on the hillsides, are commonly referred to as “Bench” gravels. Frequently there are two or more sets of benches in which case the miners refer to them as “high” benches and “low” benches.
In California and elsewhere, most alluvial deposits were quickly found by the early miners who proceeded to work the richer bedrock streaks by primitive forms of underground mining. At the time these were referred to as “hill diggings.” Following the development of hydraulic mining in the 1850’s, many of the larger bench deposits were worked by hydraulics and the smaller ones by ground sluicing. During the depression years, much of the so-called “sniper” mining was carried out on the remnants of bench gravels.