  The effective optical depth in the Ly-alpha forest region of 1061
low-resolution QSO spectra drawn from the SDSS database decreases with
decreasing redshift over the range 2.5 < z < 4. Although the evolution is
relatively smooth, tau_eff ~ (1+z)^{3.8 pm 0.2}, at z ~ 3.2 the effective
optical depth decreases suddenly, by about ten percent with respect to this
smoother evolution. It climbs back to the original smooth scaling again by z ~
2.9. We describe two techniques, one of which is new, for quantifying this
evolution which give consistent results. A variety of tests show that the
feature is not likely to be a consequence of how the QSO sample was selected,
nor the result of flux calibration or other systematic effects. Other authors
have argued that, at this same epoch, the temperature of the IGM also shows a
departure from an otherwise smooth decrease with time. These features in the
evolution of the temperature and the optical depth are signatures of the
reionization of He II.
