Four isotopes (Cs¹³⁷, K⁴⁰, Ra²²⁶, Th²³²) in superficial sediments from lakes Geneva and Morat have been analysed by γ spectrometry. The relationships have been computed between these isotopes and other chemical elements. It is shown that K⁴⁰ comes from illite while Th²³² and Ra²²⁶ are associated with clays by substitution or adsorption. Their spatial distribution is similar to that of thin detrital material. Cs¹³⁷, derived from the last nuclear explosions, is linked with the organic matter and its spatial distribution in the lakes leads to an understanding of the hydrodynamic of the basin.