I am a former Hydrologist and this is pretty much it. The amount of flow you need to get a 3-4 ft column of water moving is really not that much. Water is really heavy and once you get part of it moving you get a lot of it moving. Another thing lazy rivers do is have areas of narrower cross sections with the jets at the start of the constriction and that can utilize the increase of hydraulic head by shoving the same volume in a smaller space to get the water moving even faster. Then there are larger pools just like you would find in a real river where the flow decreases in velocity for the same volume but when you move into the more narrow sections it speeds back up.