You redirected you flow... But where is your filtration Waterfall pehaps? Might want to rethink that YOu say you canot afford a make shift greenhouse but are willing to hook up a coil heater.. I would think twice about that one.
Sounds like the problem was the wind blew the water from the waterfall and redirected it to and then the pond semi drained. The wind can also chill your pond as well and this lead to a huge drop not to mention your pond was emptied. What your trying to achieve is stable temps and keep the wind off the surface. To me a make shift greenhouse is your best bet. Check this ite ou for an inexpensive greenhouse pretty easy to put up
http://www.pvcplans.com/pvc-greenhouse.htm
SHould be fairly inexpensive to build for a 2000G pond and also to anchor it down as well...
Joe