This suggests woodworm tunneled posts and I imagine that ‘channered’ refers to the tiny channels they carve on the surface of it. This possible channel-like definition is further reinforced by another example in the same book:

‘The trickles in the mud channers and salting guts had already ceased.’

However, according to the OED, ‘channer’ is Scottish or northern Irish English, meaning ‘To grumble, complain; to chide, scold, nag; to mutter, murmur. Later also (frequently of a bird): to chatter’.

‘Between boulders and rocks crusted with shellfish and shaggy with seaweed, past worm-channered posts that marked the fairway for fishing boats at high water, the pack hunted the otter.’ 

Readers are invited to further elucidate this elusive word.

Source: Henry Williamson, Tarka the Otter: His joyful water-life and death in the two rivers, illus. C.F. Tunnicliffe (Harmondsworth: Puffin Books, 1976 (1927)), p. 178 and p. 109