American Scientist

Water may run downhill, but boaters sometimes want to go against the current. Paddles and oars might be thought of as boat lifts of sorts, in that they can be employed to raise the elevation of a watercraft from the mouth of a river almost to its headwaters. Where the river ceases to be navigable, perhaps because of shallows or rapids or a waterfall, smaller boats can be lifted physically out of the water and portaged to deeper, calmer, higher waters. In some cultures, boats have been pulled upstream against a strong current by human trackers. Horses and mules have often been used to tow boats along rivers and canals. Sails have long harnessed the power of the wind to drive a boat or ship silently, even upwind and against the flow. Steam engines, gasoline motors, diesels and turbines, although noisier, have made river travel virtually independent of winds, tides and currents.