In the process of river valley downcutting its floor width varies in time and the floor itself migrates laterally due to channel meandering. Those processes are not usually reflected in terraces series, though they affect essentially the placers evolution. The process of the valley cross-section and placer evolution is simulated by a computer. The model proved the stream downcutting regime to be independent factor of the useful component redistribution, which control the component concentration into one or several bands mostly at the valley's floor. A placer's structure depends on the valley size, downcutting duration and degree of the source preservation.