If gold turned up, the miner worked his way upstream and eventually uphill to find the source. The material in the pan is noticeably darker than the surrounding gravels. Here's where it all began, Coloma, California. The gold had weathered out of rocks upstream. Below are views looking up and down the American River here.
Placer mining requires large amounts of water.
In some gold-rush areas of California, nearly every back road is flanked by a small canal. The Malakoff Diggings over a century after mining ceased. The flat upland surface is an old erosion surface covered by Tertiary lava flows. Many of these flows moved down ancient valleys and buried ancient stream gravels that were also gold-bearing. After attacking modern gravels, gold miners went after the ancient gravels as well.