I'm not positive about whether by "runoff" DD is referring to water flowing into your pond (I think that is what he is talking about, but when Dave starts talking Texan nobody but the Texans really know what he's talking about) but that's besides the point. The point is you need to be concerned about two types of water movement. Water flowing into your pond, and excess water flowing out of your pond. Both will have a significant impact on your pond. If the water flowing into your pond is contaminated with fertilizer or cow chips or nuclear waste or something something like that it can have a significant impact on your pond. (Excess nutrients from fertilizer can cause excess algae blooms).
I highly recommend that you down load and read this publication.
USDA Agriculture Handbook # 590 "Ponds - Planning, Design, Construction" It is a wealth of information. Also order and read Bob Lusk's book "Perfect Pond, Want One?", another must have pond builders guide!
Hang on for some better answers and a Texas to English translation.
