It can be difficult to ID Western from Clark's Grebe in the fall when the face pattern is not as distinct. Multiple references (Sibley, Kaufman's Advanced Birding, Kim Eckert's article in the Oct 1993 Birding "Identification of Western and Clark's Grebes," and one of the seminal early papers (from a group at U of M no less) published in The Auk supporting a split of the two forms) suggest that bill color is the best way to separate the two birds; Western has a yellow bill tinged greenish, Clark's has a yellow bill tinged orangish. These are my two best pics of the bill when it's not reflecting direct morning sunlight, certainly not tinged orange. Also note the blurry duskiness to the upper and lower mandibles, another Western feature (Clark's has a more sharply defined black edge to the upper mandible):
The next 2 pics show the face pattern (note that the bill is reflecting directly off the sun making it seem very bright; Kaufman specifically cautions that a Western's bill can look very bright in low direct sun). This bird shows a solid black cap. The lores, as well as a narrow line of feathers below the eye, and the feathers directly behind the eye are evenly blackish, paler than the cap. While the two species have very little overlap in breeding plumage (see the very neat Figure 2 in the Auk paper linked above), there is some overlap in non-breeding plumage. However Clark's still usually has white lores and indeed the extent of black on the birds face looks to me to fall in with that figure's group 2 in which they had no birds with Clark's type bills.
Here's a comparison with 2 breeding Western Grebes from my photos, the left bird (with a chick just visible on its back) from South Dakota and the right bird from Colorado. Even in presumed non-breeding plumage, our bird's face looks not appreciably different from these breeding birds.
Here's two views of the bird going away showing how broad the black is down the back of the neck:
Here's a comparison from Colorado of a Clark's Grebe on the left and Western on the right, the difference then with the two side by side was marked, though not as well emphasized in this pic since I couldn't get them exactly parallel:
In this pic you can also see the 3 gray-toned look that Clark's can show, with a black cap and back of the neck, med-dark gray back and light gray sides. Our bird did show some mottled whitish feathers on the rear flanks especially along the waterline towards the stern, but the overall color of the side panel was still relatively dark. The Birding article's exhibit photo of a typical Western Grebe shows a bird with some mottled white feathering towards the rear of the flanks. Kaufman and the two articles all point out that the flank coloring is variable both between birds and between views of the same bird; while Sibley points out Clark's averages more pale, I still think this implies that overlap occurs. Interestingly, Sibley also emphasizes extensive white flight feathers in Clark's, the other references say there's almost complete overlap or do not even mention this point at all.
While if I'd had the opportunity I would have stayed with the bird longer, and may look for it again on Monday, in my mind this is a Western Grebe.