Question
We live in Chicago.  We have two ponds (each 3,000 gallons)  Water is
pumped from the lower pond to a waterfall in the the upper pond.  The water
then  flows down a stream from that pond back in to the lower pond, and the
process repeats.  There are filters in each pond.

There are fish in both ponds and they have all survived the winter.  We
turned all pumps off over the winter and kept an opening in surface with
heaters.  

We would like to turn the pumps on but want to know if that will put the fish
at risk in any way.  The fish in the lower pond have been more active than the
upper fish.  Maintenance of the ponds isn't scheduled until 2-3 weeks.  
Today and tomorrow will in 60 - 64 degrees and next week 50's during the
day and 36'+ at night.

Actually, my husband has just reported that he has turned the pumps on so
this may be a question that is moot, but please let me know your thoughts.  
Thank you for your time.

Answer
Hello Linda!
Turning the pumps on in a pond should not generally hurt the fish, but there are exceptions.
Pond Fish, and Koi in particular, are sensitive to sudden temperature changes. Thus, when the pumps are turned on on a day when the water may have stratified (developed much higher temperatures in some layers of water due to outside temperatures) and the fish are suddenly subjected to significantly different temperatures, they can come to harm and even death can result.