The area of land that supplies water to a point in a river is called a watershed. For example, the image below shows how far the water might travel to reach someone standing next to the White River in 100 Acres.

Rain that falls on these 1,265 square miles eventually finds its way to the Park and flows down the White River to the Mississippi, and then to the Gulf of Mexico where it enters the ocean. This is just one part of the hydrologic cycle.

Land use and pollution

During the summer large amounts of green algae, called algal blooms, can accumulate in ponds and streams. Algal blooms are caused when large concentrations of nutrients such as phosphorus are present in a body of water. This map shows how large amounts of nutrients can get into the water.

As seen here, the primary land use in the watershed that feeds 100 Acres is agriculture. Some of the nutrients from the fertilizers that farmers use to grow crops are carried by water into streams and eventually flow past the Park and downstream to Indianapolis and beyond. The second largest land use in the watershed is urban areas. When it storms, fertilizers from lawns are also washed over the ground, through storm drains, and into our rivers.

Impervious surfaces

In urban areas, there are many surfaces that are less permeable than soil. Roads, parking lots and rooftops can't absorb moisture (and are, in fact, designed not to) while restrain runoff like soil and foliage can. As a result, during a storm in an urban area there is a larger percentage of precipitation that flows over the ground and into rivers than there would be if the area were undeveloped. This chart (called a hydrograph) shows the difference between Mercer Creek (an urban stream) and Newaukum Creek (a nearby rural stream) during the week after a storm on February 1, 2000. The urban stream has a more rapid climb and higher peak for the volume of flow than the rural stream.

In regions affected by flash flooding, some communities promote landscaping improvements, such as the creation of rain gardens, which can capture some of the runoff that would otherwise contribute to flooding. As an added bonus, such improvements can filter pollutants as well.

Learn more about watersheds:

• U.S. Geological Survey, Water Science for Schools – What is a Watershed?
• U.S. Geological Survey, National Water Quality Assessment Program, Brief description of the White River Basin
• U.S. Geological Survey, Hydrologic Unit Maps
• U.S. Geological Survey, The effects of urbanization on water quality, urban runoff
• Environmental Protection Agency, Pollution Prevention: Water Conservation