Geology: Watersheds

Have you ever stood next to a river and wondered where is all of the water coming from? Well, the area of land that supplies water to the point where you were standing is called a watershed.

More specifically, if you were standing next to the White River in 100 Acres, the image below shows how far that water might have traveled to reach you. 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

Have you ever noticed large amounts of green algae in ponds and streams during the summer? These are called algal blooms, and they are caused when large concentrations of nutrients such as phosphorus are present in a body of water. If you are interested in how large amounts of nutrients get into the water, take a look at the map below.

As you can see, 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. You might also notice that urban areas are the second largest land use in the watershed. 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) or 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. The chart below (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. Notice the more rapid climb and higher peak for the volume of flow in the urban 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.