The simple answer is that a watershed contains all the land and water features that drain excess surface water to a specific location on the landscape. In other words, standing on the land and looking around, everything uphill from that position routes water to that point and falls within its watershed.
The definition of a watershed is scope and scale independent. For example, a small pothole wetland surrounded by an immediate divide will receive limited overland flow. Some wetland watersheds are only a few acres in size.
In contrast, a lake like Lake Pepin, on the Minnesota-Wisconsin border, drains much of Minnesota including all lands that drain to the Upper Mississippi, St. Croix and Minnesota Rivers. Its watershed is more than 30 million acres in size.
Both of these meet the same definition of a watershed. The only difference is the amount of land that contributes overland flow to the selected point of interest.
For the purpose of the Watershed Health Assessment Framework, the MN DNR Major Watershed Boundaries (sometimes referred to as HUC (Hydrologic Unit Code) 8 Watersheds) are used to summarize the health scores and other information. This decision was made while recognizing that many other watershed scales exist and in most cases a Major Watershed is not a "true" watershed but rather a delineated boundary within a larger watershed.
For the purpose of this analysis, Major Watersheds were selected because:
A complicating factor in watershed modeling is the movement of water below the surface. Groundwater flow is not limited by the topography of the land above ground and may flow across watershed divides that occur on the surface.
Watershed boundaries may change over time. Watershed divides may shift due to natural factors such as erosive action, or man made interventions such as dams, culverts, roads and drainage structures. Watershed delineations are updated as newer technologies more accurately capture very small changes in surface elevation and subsequent water routing.