Depolarized low-frequency Raman spectra of aqueous electrolyte solutions (ACl and BCl2; A=Li, Na, K, Rb and B=Mg, Ca) in the frequency region from -250 to 250 cm(-1) have been investigated over a wide concentration range. The spectra were analyzed with one Cole-Cole type relaxation mode and two damped harmonic oscillators. A broad relaxation mode was found below 20 cm(-1) For the present aqueous electrolyte solutions, with increasing salt concentration each relaxation time tau becomes longer than that of liquid water tau(water). The relaxation time is considered to correspond to the duration time of the tetrahedral structure of bulk water. We have found that the concentration dependence of the ratio tau/tau(water) is equivalent to the ratio eta/eta(water) where eta is the viscosity of aqueous solutions.