keeping the pond from freezing over

With a high temperature of about 15 degrees F today, our little back yard pond is one of the few places around with open water. Many birds (and some larger animals) are drinking from it. The pond is 4-5 feet deep - deep enough that it will never freeze at the bottom in the Boston area. We keep a pump running (on a two foot deep ledge on the side of the pond) to keep the water from freezing. I just went out and repositioned the pump so that the water outlet is aiming upward at the pond's surface. If we have a very long stretch of cold weather (more than a week in the teens), or a lot of snow that covers the surface, we float a pond heater on the water. I've kept koi year round in this pond for the past 5 years. Right now they are buried down in the leaves and muck at the bottom and look quite happy. Brrrr!