A stream that flows between 6 mph and 9 mph are adequate. They can also operate in as little as 1 foot of water. Ram pumps have been around for a long time and are quite interesting to say the least. Ram pumps can be used for irrigation, gardening and bringing water up to a storage tank. Ram pumps are commercially available or you can even build one yourself.
Water accelerates through a pipe and open waste valve.
It's velocity increases until the flow and upward force causes the waste valve to slam shut. When the valve slams shut it momentarily produces a short-lived pressure that causes a small amount of water to be released through the check valve into the pressure tank. At this point, the pressure below the check valve is lower than the pressure in the tank and the check valve releases and the water is pumped out. This cycle repeats continuously as long as there is water flow. Although a ram pump doesn't deliver a large volume of water, it's continuous and even the largest of storage tanks will fill up with water over time.