jlaw
I would think a little research is needed here. Here in Canada streams can freeze up and at the same time the ground water 6' below is still 50 Deg F. You maybe subjected to large temperature swings with a stream running basically across the ground exposed to the elements. Deep ponds or rivers a different story. There maybe ice on the surface but deeper could be several degrees above the freezing level.
That being said a stream as you say is running continueously all year is better at 34 deg F than trying to get heat out of air via ASHP at 20 DegF But still your COP will not be consistantly good.
If your going to purchase a water to air heat-pump anyway and bring in an excavator to bury the tubing a foot below grade. I would suggest going the extra few feet and place your ground loop at the proper depth for your area and have all the benefits of the ground source heat pumps.
The new heat-pumps are so energy efficient with a good install you would be so pleased.
If your going to hack your own heat-pump the ground loop is an extremely important part of the system. The better the source of heat the better overall performance.
Randen
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