This picture shows a mixture of Ross's and Snow Geese; when they are seen side by side, the features that distinguish them are readily noticed. All six of the birds still in flight are Snows; most of the birds already in the water are Ross's, a few are Snows. Snow Geese are larger, have proportionally larger bills and flatter heads, and the bills show a prominent dark "grinning patch." They also often (as in this picture) have orange-tinged heads, with the usually white feathers stained by iron ore in the mud into which they thrust their heads while feeding. Ross's do not show these iron stains.

SEE http://www.pbase.com/image/148376205 for a full-size version of this picture, allowing a closer look at the individual geese, so the differences between the two species can be more clearly seen.