You're correct that rain alone cannot supply all the water that flows over waterfalls like Niagara Falls. The water cycle, also known as the hydrological cycle, is the process by which water is continuously circulated on Earth. It involves various mechanisms that transport water from the surface to the atmosphere and back again, replenishing water sources such as rivers and waterfalls.
In the case of waterfalls, the primary source of water is usually rivers or streams. The water in these rivers originates from various sources, including rainfall, snowmelt, underground springs, and other forms of precipitation. As the river flows downstream, it gradually gains volume and accumulates more water from its tributaries along the way.
When the river encounters a steep decline in its path, such as a cliff or a sudden change in elevation, it forms a waterfall. The water plunges down from the higher elevation to the lower elevation, creating the spectacle of a waterfall. This process is driven by gravity, as the water seeks the lowest point available.
Once the water reaches the bottom of the waterfall, it continues to flow downstream as a river or stream. It eventually undergoes evaporation, condensation, and precipitation again, which can occur over the same region or in a different area. This replenishes the water sources and sustains the water cycle.
While rain plays a part in the water cycle, it is the overall process of evaporation, condensation, and precipitation that ensures the continuous circulation of water on Earth. The movement of water from high up in waterfalls to lower elevations and its subsequent return to higher elevations is a natural outcome of this cycle, driven by the Earth's energy balance and gravitational forces.