Tam heads for the River Doon, because, according to folklore, witches cannot cross running water.

Tam galloped across and saved his life, as witches cannot cross water.

"They think witches can't cross a door painted blue or green."

Finally the witch crossed herself, laid the papers back in the envelope, and rose to stretch cramped muscles.

When he had done this there was a great lake behind them, and this the witch could not cross until she ran home again and brought her dough-trough.

I recall that witches cannot cross water.

The witch crossed the room to rip open that panel where Koris had stored the ax.

Knowing that witches can't cross water, he heads for the Brig o' Doon.

The witches cannot cross the water but they come so close to catching Tam and Meg that they pull Meg's tail off just as she reaches the bridge over the Doon.

I think witches cross anything they like.