I have never understood this passage.  Word Fans, can you help me out?

[14.012] Amid shrieks and wailing and the shouts of men he came over them, swept towards the bridges and was foiled! The bridge was gone, and his enemies were on an island in deep water – too deep and dark and cool for his liking. If he plunged into it, a vapour and a steam would arise enough to cover all the land with a mist for days; but the lake was mightier than he, it would quench him before he could pass through.

Sure, Smaug didn’t want to fall into the lake – I am pleased and surprised to learn that the lake was mightier than he – but couldn’t he just fly over?

• 14.012 and was foiled!