A watershed is the landscape area that contains all the land and water features that drain surface water to a specific location. In other words, as you stand and look around, everything uphill that routes water to that point is part of its watershed.Each watershed is based on a unique pour point location, through which all surface water leaves that watershed. Often a pour point is located at a confluence where two streams or rivers flow together.A watershed can range in size from a few acres around a small pond, to an entire river basin. All that matters is where you stand when you ask the question, "What land is uphill from this point?"
Administrative versus true watersheds
In practice, natural resource professionals delineate watersheds so that they create a complete coverage over the landscape, and each watershed is a similar size. To accomplish these objectives, some watersheds will not contain all of their upstream area.These functional watershed boundaries are called administrative watersheds. In contrast, a watershed that does contain all of its upstream area is called a true watershed.The watershed outlined in white is an administrative boundary for the Brainerd Major Watershed. Note that, there are several streams flowing into this watershed from the north. The watershed outlined in pink is a true watershed that contains all of the area contributing surface water. The two watersheds share the same pour point, which is located at the orange diamond.
The next sections describe three administrative watershed scales that are commonly used in Minnesota.
River basins
In Minnesota there are 12 river basins, shown outlined in blue in the map below. Only the Upper Mississippi River Basin, located in the center of the state, lies entirely within Minnesota. All the other river basins include some land area beyond the state boundary.The river basins are also referred to as HUC (Hydrologic Unit Code) 4 watersheds.The river basins are a reminder that Minnesota is in a unique position. Minnesota holds the headwaters for several large river systems. This means that problems in our river systems start right here. This also means that when we improve Minnesota Rivers, those benefits flow far downstream.
Major watersheds
A major watershed is the administrative boundary most commonly used for management and assessment of resources. Like pieces of a puzzle, major watersheds subdivide the river basins into smaller watershed areas.There are 80 major watersheds in Minnesota, outlined below in white, subdividing each river basin. The major watersheds are also referred to as HUC 8 watersheds.Almost half of Minnesota's major watersheds are not 'true' watersheds but rather a subsection of a larger true watershed.
Catchment watersheds
Inside of each Major Watershed, there are smaller puzzle pieces. These subdivisions are called catchments, catchments shown in light gray within a few major watersheds in north central Minnesota. These smaller watersheds help managers understand how water flows and connects at local scales.For example, high points on the landscape form headwater areas. A catchment in this location will capture water and send it downstream. If your location is lower down near the mouth of a river, you will be receiving water that has already traveled through many other catchments.
Watersheds can change
Watershed boundaries are not static lines on the map, but rather a representation of the way we think water is flowing. New information can improve our understanding, influencing where we draw those lines on a map.A complicating factor in understanding watershed boundaries 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 also updated as newer technologies more accurately capture very small changes in surface elevations that route water. This image shows a high-resolution hillshade using LiDAR technology that helps managers more precisely determine how water moves across the landscape.
Why manage watersheds?
A watershed is much more than a boundary on a map. Within a watershed, natural processes and human activities are connected in important ways. A change in how land and water are managed in one place can create a response in another location.For example, the location of natural vegetation like woods, prairie and shrubland will influence where animals can live and move, how much water will flow downstream, and whether soil is likely to erode.
Explore more
Visit the MN DNR Watershed Health Assessment Framework (WHAF) website for more about Minnesota's watersheds and how we measure and characterize health for watershed systems.Check out our interactive WHAF: Explorer map which provides tools to compare watershed scales and health conditions for any location in Minnesota! In this map, the upstream scale is already masked to the true watershed. Click a new location to explore other true watersheds in Minnesota.
Want more about defining and delineating watersheds? The History and Standards of Watershed Delineation describes the science that determines the boundaries.