This note presents observations of sound wave backscattering in a turbulent water flow, conducted with a frequency scanning Doppler sonar working in a 0.5 to 5 MHz frequency band. The observed sound wave backscattering is attributed to temperature microstructures created by water turbulence. The backscattering intensity decreases systematically as a function of sonar frequency above 1 MHz and falls below detectability for sonar frequencies higher than a given value function of water turbulence intensity. This backscattering cutoff relates to the high wavenumber end, kB, of the turbulent temperature spectrum. This kB estimate is found in excellent agreement with its theoretical calculation based on independent measurement of turbulence dissipation rate made using the Doppler observed velocity variance. ¿ American Geophysical Union 1993