Horizontally distributed temperature time series recorded in the thermocline of a small reservoir during autumn are used to identify both basin-scale and higher-frequency internal waves. Stationary and nonstationary spectral analyses are used to identify dominant frequencies. The horizontal distribution of data allows a map of the basin-scale wave energy to be determined. Three-layer normal mode modeling incorporating damping emulates the basin-scale behavior well. This provides estimates of velocity shear and hence the internal stability. The nonstationary frequency analysis identifies times of high-frequency internal wave activity (wave periods of ~2--10 min). This is definitely linked to the basin-scale wave-driven shear and hence, to decreases in stability. Estimates of dissipation for the thermocline and hypolimnion are 3¿10-8 and 6¿10-9 m2 s-3, respectively. The inferred variability in the hypolimnetic value is demonstrated. ¿ 1999 American Geophysical Union