h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I think (having thought for some time about this) that I'd like to nominate the section of the Afon Glaslyn known at the Glaslyn gorge near Beddgelert in North Wales.
On a hot summers day it's an idyllic walk through a narrow mountain pass, river on one side, trackbed of the old Welsh Highland Rainlway on the other. Trees dropping down to the river, rocks exposed with the river dropping between them, buzzards circling overhead. Children playing swimming in the river and sliding down the waterfalls. With a pleasant walk back to the town of Beddgelert and a cracking ice cream to look forward to.
Having a look again in winter will reveal a rushing torrent of water trying to force it's way down from the mountains of Snowdonia to the sea. Where rocks were uncovered in summer now they've got water flowing over and around them. The small waterfalls have now become much greater in volume and the noise and spray are noticeable. Pools between drops now have eddies flowwing round and round as the water is forced into circulating. Turn up sometime in the winter and you might see some people kayaking down this section of river.

Of course, within the next few years the Welsh Highland Railway should be running through the gorge once more...