Your first question: The water runs down the slope
Your second question: The rock recedes means that base of the mesa erodes and moves back. Imagine you put a stick in the ground against the base of the mesa. Now wait 1,000 years and go back. The stick would still be in the same place, but the base will have moved back and won't be touching the stick anymore -the base will have "receded".
I hope that helps.