Simultaneous profiles of microstructure, horizontal velocity, and acoustic backscatter allow one of the most complete descriptions of a naturally occurring shear instability to data. Shear increased rapidly after passing through a lateral constriction which formed a hydraulic control. A kilometer-long set of 20-m-tall billows grew on a middepth density interface where the Richardson number fell below 0.25. The velocity interface thickened steadily after the billows formed, consistent with rapid momentum mixing across a shear layer with a Reynolds number of 3¿106. The billows generated large density overturns and dissipation rates greater than 10-5 W kg-1, even within the first large overturn, indicating that these structures were fully turbulent early in their development. As the billows grew, a well-mixed layer developed at the interface and survived as an actively turbulent layer for up to 6 buoyancy periods, 3 times longer than in laboratory studies at low Reynolds number. Variations in the mean density of the billows lead us to infer that the vertical offset of the velocity and density interfaces varied with time where the billows first formed. With data from the large overturns within the shear layer, we find &egr;/&ngr;N2≈3¿104, an average root-mean-square overturn scale (L¿rms) of 2.6 m, and a buoyancy scale (Lb) of 2.7 m. Despite having sampled the billows at varying stages of their evolution, we find no indication that the ratio Lrms/Lb is ever significantly different than 1 for this shear instability. ¿ American Geophysical Union 1994