By the example of the Ingoda basin (Transbaikalia) including a system of small lakes, the role of two leading processes of concentration of trace elements in small reservoirs (water–rock interaction and evaporation) has been estimated. It is shown that the accumulation of U, As, Se, Sb, and Ge does not depend on the kind of concentration. Their contents increase linearly with total dissolved solids in both lake and feeding waters. Accumulation of Sc, V, Y, Th, REE, and W is mainly due to evaporation, with their maximum concentration by three to four orders of magnitude. Only U, W, and REE, whose contents in the lakes reach n⋅100 μg/L, are of practical interest.