Green Go,
Any pump can have the output flow of the pump adjusted by installing a ball valve on the output.
Mike

I use the above method on diesel fuel systems for 20 years and it's a good solution, would suggest that you don't close the valve below 50% so you don't overly increase the back pressure on the pump.

Most pond pumps are low pressure high volume pumps. So the only problem you can run into with these pumps is heat. Submersible pumps use the water to cool them off. If they are in a closure they can heat the water in the closure if not enough water is flowing in to cool them off. As long as the water in the closure is the same temperature as the pond water the pump will be fine no matter how much the valve is closed. Most submersible pumps use about the same amount of power no matter how much water they are moving. External pumps are a little different. As long as the water in the pump is staying cool the output is fine and won't hurt the pump. External pumps use less power the lower the amount of water they are moving. All pumps will last longer with some back pressure. Normally the head pressure of the piping and the height that water is being lifted is normally enough if the system is sized correctly. If there isn't enough back pressure the motor on the pump will run hotter than it was designed to operate at. The worse thing that a pump can see is for there to be more head pressure on the intake side than on the output side. This normally can only happen to external pumps.
mike