These are awesome, keep it up!

These are really nice, and also work well for someone who is starting out doing their own isometric graphics, especially the hills.

However I'm unsure about the uphill/downhill river - maybe something like a river lock would be a better graphic to use? And for higher drops, a waterfall.

In addition, perhaps it would be good to separate the land forms from the constructions opon the land, i.e., the road graphics, river graphics, rail graphics, buildings, trees, etc would all be separate from the underlying land block. This means you can have multiple land blocks (grassy, stoney, desert, etc) without having to redo all the constructions upon that block.