  We present sub-arcsecond (0.35"-0.9"), near-infrared J,H,K band photometric
observations of six fields along the W51 Giant Molecular Cloud (W51 GMC). Our
observations reveal four new, embedded clusters and provide a new
high-resolution (0.35") view of the W51IRS2 (G49.5-0.4) region. The cluster
associated with G48.9-0.3 is found to be a double cluster enclosed in a nest of
near-infrared nebulosity. We construct stellar surface density maps for four
major clusters in the W51 GMC. These unveil the underlying hierarchical
structure. Color-color and color-magnitude diagrams for each of these clusters
show clear differences in the embedded stellar populations and indicate the
relative ages of these clusters. In particular, the clusters associated with
the HII regions G48.9-0.3 and G49.0-0.3 are found to have a high fraction of
YSOs and are therefore considered the youngest of all the near-infrared
clusters in the W51 GMC. The estimated masses of the individual clusters, when
summed, yield a total stellar mass of ~10^4 Msun in the W51 GMC, implying a
star formation efficiency of 5-10%. These results in comparision with the CO
observations of the W51 GMC, suggest for the first time, that star formation in
the W51 GMC is likely triggered by a galactic spiral density wave.
