Not only will the cold water (if in enough volume) drastically change the water temp in the pond, but if it isn't run over something to agitate the water as it goes into the pond, it will drastically reduce the O2 levels in the pond as well. Well water is mostly devoid of O2. It will rapidly pick up O2 from the air, but not if it's pumped directly into the water that is already in the pond. It needs some type of exposure to air to pick up the O2, and the more well water that is exposed to the air, the faster that it picks up the O2.
Changing the pond temp all depends on the quantity of water left in the pond, and how fast the water is pumped in. I didn't notice any temp change when pumping 28 gpm into a 1 ac pond other than right close to where the water was entering the pond.
