  We report time-resolved x-ray scattering measurements of the transient
structural response of the sliding {\bf Q}$_{1}$ charge-density-wave (CDW) in
NbSe$_{3}$ to a reversal of the driving electric field. The observed time scale
characterizing this response at 70K varies from $\sim$ 15 msec for driving
fields near threshold to $\sim$ 2 msec for fields well above threshold. The
position and time-dependent strain of the CDW is analyzed in terms of a
phenomenological equation of motion for the phase of the CDW order parameter.
The value of the damping constant, $\gamma = (3.2 \pm 0.7) \times 10^{-19}$ eV
$\cdot$ seconds $\cdot$ \AA$^{-3}$, is in excellent agreement with the value
determined from transport measurements. As the driving field approaches
threshold from above, the line shape becomes bimodal, suggesting that the CDW
does not depin throughout the entire sample at one well-defined voltage.
