Watersheds, or drainage basins, are areas of land where all the water under it and all the water that drains off of it ends up in the same place. Oftentimes sediment, pollution, and fertilizer runoff can be deposited and concentrated in their watersheds, which is why it's important to remember to be cautious when dealing with substances that may end up in the groundwater.
The Hydrologic Studies Unit of the Land and Water Management Division in the Michigan Department of Environmental Quality made this helpful map to let people know which major watershed they were in.
Washtenaw County is mostly in the Huron watershed, but fairly large parts of it are also in the Raisin and Stony Creek watersheds.
See a larger and more complete version (including the U.P. and the legend) of the map here, and read more about watersheds here or here.