Bodies of water are fed by groundwater and rainwater from the surrounding area. That area is known as a watershed.
The Gull Lake Watershed spans two counties and four townships. Needless to say, what happens anywhere within this area can and does impact Gull Lake and the other lakes, streams and marsh areas.
This map is based on work done by Dr. David P. Lusch of the Michigan State University Center for Remote Sensing and Geographic Information Science back in 2000.