  We report on the depletion and power amplification of the driving laser pulse
in a strongly-driven laser wakefield accelerator. Simultaneous measurement of
the transmitted pulse energy and temporal shape indicate an increase in peak
power from $187 \pm 11$ TW to a maximum of $318 \pm 12$ TW after 13 mm of
propagation in plasma density of $0.9 \times 10^{18}$ cm$^{-3}$. The power
amplification is correlated with the injection and acceleration of electrons in
the nonlinear wakefield. This process is modeled by including localized
redshift and subsequent group delay dispersion at the laser pulse front.
