  Impurity pinning has long been discussed to have a profound effect on the
dynamics of an incommensurate charge density wave (CDW), which would otherwise
slide through the lattice without resistance. Here we visualize the impurity
pinning evolution of the CDW in ZrTe3 using the variable temperature scanning
tunneling microscopy (STM). At low temperatures, we observe a quasi-1D
incommensurate CDW modulation moderately correlated to the impurity positions,
indicating a weak impurity pinning. As we raise the sample temperature, the CDW
modulation gets progressively weakened and distorted, while the correlation
with the impurities becomes stronger. Above the CDW transition temperature,
short-range modulations persist with the phase almost all pinned by impurities.
The evolution from weak to strong impurity pinning through the CDW transition
can be understood as a result of losing phase rigidity.
