Some of the biggest threats to water’s quality are the modifications or changes we’ve made to the landscapes that water moves through. Explore More about the ways we’ve interfered with the natural hydrologic cycle and the effects of those changes.

The Cycle of Life
Whether you notice it or not, water is constantly on the move all around us. It’s falling to the land in the form of precipitation. It’s moving across landscapes making its way to streams. Streams are running to rivers and rivers are running to oceans. Water is evaporating and transpirating and absorbing. Water is the ultimate recycled element thanks to the hydrologic cycle, which constantly moves water from the atmosphere–down to the planet’s surface and then back into the atmosphere.

The Natural Water Cycle
In a natural system, with no human modifications, water "percolates" through many different elements (sand and dirt, rocky and grassy areas) before reaching waterways like streams and rivers. Those elements help filter out pollutants. Sometimes they act as holding areas, creating standing water waiting to go through the system. Humans have made hydrologic modifications short—working around this natural flow, forcing water more directly into waterways, and making the water more vulnerable to pollutants.

Human Modification
Modifications to the natural landscapes, like tiling and uprooting prairies, were made for many different reasons. Some were made to support agriculture; forests were cleared and lands were drained. Other modifications supported development of cities and towns, resulting in more impervious surfaces and less pervious ones. Major changes, like shifting a river’s course or straightening its flow, were made to support navigation. Whatever the reasons for the modifications, they had the same outcome—changing the way water moves through landscapes, affecting what water comes into contact with and ultimately, water’s quality.