Not to worry. That's actually a common problem. You could combat that problem by either building a cinder block collar, or by pouring a concrete collar around the edge of the pond. If the pond is on the small side (which it sounds like it is), I'd recommend the concrete collar method. Just be sure to use rebar to reinforce the concrete and that will take care of your problem. If you look at my post "how to build a koi pond step by step," you'll see how I built a cinder block collar for my pond.
Sorry I took so long to post.