Back to front (matching the direction of the river flow), bottom up.
The initial map will fade in from a blank white page, possibly starting with some live video of someone unrolling a blank "map" on a desk/table. That fade will use the elevation data as well, but of course the terrain will still be flat. Right to left for that one and haven't decided whether to go high to low or vice versa. Conceptually I can see both working so I'll probably wait to see how they look. Given the "low" accuracy of the first map I'll use a 100m DEM for that mask.
The initial surfacing will be static and when completed I'll "start the clock" and get the clouds and sun moving.
Saving up for a scan so I can make a cameo appearance in a fly by over a ridge/cliff edge
Hoping to do 1920x1080 but the frames with surfacing will take longer to render. The DEMs and image maps are pretty quick (750 frames @ 1080px wide on 2 PCs in <12 hrs)