Much work has been done on ocean mixing driven by shear instability, which predicts a correspondence between turbulence and low Richardson number. However, this correspondence is not often seen. I’ll first review theory and observational evidence for breaking internal gravity waves due to shear instability and convective instability. Then I will present recent observations in a submarine canyon demonstrating onset of turbulence many buoyancy periods (hours) after unstable Richardson number, which turns out to be consistent with numerical simulations of parallel shear flows. The implication is that there is yet much to learn about the time history of turbulence under time-dependent forcing conditions.