The Power of Moving Water

The hydrologic cycle brings continuous supplies of freshwater

to the uplands that feed river systems.

All around planet Earth, water is on the move. In rivers and creeks, water fl ows downhill under the force of gravity. It starts off as rain or snow falling on the highlands and mountains. Running water forms tiny rivulets and streams, which gather to form large rivers. Most rivers find their way to the edges of the continents, where they dump massive loads of fresh water and sediments into the oceans. Evaporation from the surface of rivers, lakes, and oceans brings the water back into the atmosphere as invisible water vapour. Under the right conditions, unseen water vapour condenses from the air to form clouds and possibly rain, snow, or hail. Seasonal rain and snowfalls bring fresh water back to the headwaters of streams, completing a very important ecological system called the “hydrologic cycle.” By bringing fresh supplies of water to the highlands, the hydrologic cycle ensures that we always have energy available from flowing water.
Rivers and streams are among nature’s most powerful forces. The force of water moving down a moderately-sized river can exceed several million horsepower. Over time, this force can slice through mountain ranges, and haul billions of tonnes of soil and debris to the oceans. This is the force humans attempt to harness when they build dams to generate electricity.

Moving water is one of nature’s mightiest forces.

Rivers are the most familiar form of water in motion, but there are others! Ocean waves, tides, and currents move unimaginable amounts of water around every day. Currents and waves are usually caused by winds blowing over the surface of the ocean, while tides are caused by the moon’s gravity pulling gently on the earth. The action of waves, tides, and currents is especially noticeable near coastlines and islands, where they cause signifi cant erosion.
Moving water is an important source of mechanical energy. Water is very dense compared to air, and flowing water carries with it far more energy than a similar volume of moving air. Humans have long appreciated the power of moving water, and have been using it for thousands of years.