What is a watershed?

A watershed is an area of land that drains all the surface water and precipitation to a common outlet/catchment area such as a lake, bay, or river. The common catchment area for the Lake Auburn Watershed is Lake Auburn. The watershed is 11, 758 acres and Lake Auburn is 2,298 acres. Not including the lake, the watershed is made up with different types of land uses. Developed land is 15% of the watershed, agriculture land is 6%, forest land is 74%, and water and wetlands is 5% of the watershed. Water in the watershed follows the topography, or features of the land. These features can be natural such as mountains, hills, ponds and streams, but they can also be man-made such as housing complexes and roads.


Surface Runoff

Surface runoff is water from precipitation or another event, flowing over the landscape of the watershed.

Roads, buildings, compacted soil, and other hard surfaces are impervious surfaces. These are surfaces that prohibit the absorption of water, causing the water to speedily flow over the watershed. In comparison, permeable surfaces are porous surfaces that allow water to move and filter through them, they work in a similar way that a kitchen sponge can absorb and hold water. Most vegetated areas are permeable surfaces.

Why is this important? Water easily picks up pollutants that come in contact with the water. When surface runoff moves fast over impervious surfaces, it travels fast into Lake Auburn, delivering the pollutants it picked up directly into the lake. Lots of surface runoff also disturbs the natural water cycle by having significantly decreased evapotranspiration and groundwater. When water travels over permeable surfaces, it is absorbed into the soil. The soil, vegetation, microbes, and fungi in the area start working on naturally separating out the pollutants from the water, and then breaking those pollutants down. Lake Auburn is the drinking water supply for over 41,000 people in the Lewiston and Auburn area. Native vegetated areas are vital in our watershed for their ability to slow the flow and filter out pollutants from our water.
As the percentage of impervious surfaces increases, the percentage of surface runoff increases, and the percentage of infiltration and evapotranspiration decreases. With higher percentages of impervious surfaces, there are just less opportunities for water to enter the infiltration and evapotranspiration parts of the water cycle.

Adding vegetation or engineered permeable surfaces to urban areas helps mitigate surface runoff. The image above describes different ways that individuals or cities can improve impervious surfaces. The closer we model a natural undeveloped watershed, the healthier our own watershed will be.

For more information check out Maine Lakes’ handbook The Lake Book: A Handbook for Lake Protection. This book is a wide range of information about lake science, health, and conservation in one easy-to-read, informative resource.