Choose a waterfall pump to match the height and width of your waterfall. The rule of thumb is for every 1 in. of weir width, you need 100 gallons per hour flow to achieve a “sheet” look.
For a "trickle" look, divide by two.
For a "Niagara" look, multiply by two.
Be sure to check the pump’s specification for the ideal pumping height of your pump at the head of your waterfall. Keep in mind, excessive bend or in-line filters will reduce the flow.