Understanding Watershed Boundaries and Drainage Maps

What Sets a Watershed's Boundary

A watershed's boundary is set by the shape of the land. The boundary, called a drainage divide, follows the highest ground that separates water flowing toward one outlet from water flowing toward another. Rain or snowmelt on one side of the divide runs downhill toward one stream, lake or wetland. Water on the other side goes somewhere else.

Every watershed is defined for a particular outlet, also called a pour point. This is the spot downstream where all the water from the area leaves or collects. If you move the outlet, the watershed changes. A watershed drawn for a small creek's mouth is a small piece of a larger watershed drawn for the river that creek joins. That is why watersheds are described as nested. Even large river basins are built from many smaller drainage areas.

Why Prairie Divides Are Harder to See

In mountains, divides follow obvious ridgelines. On the northern Great Plains the land can look nearly level, and the divide between two drainages may be a rise of ground that is barely noticeable on foot. Several features of glaciated prairie, including the landscape around Medicine Lake in northeastern Montana, make the boundaries less tidy than textbook diagrams suggest.

• Closed depressions: Continental glaciers left a hummocky surface dotted with shallow basins, known as prairie potholes. Many potholes have no surface outlet. Water that reaches them stays there until it evaporates or soaks into the ground.
• Noncontributing areas: Because of those closed basins, parts of a mapped watershed may not send surface water to the watershed's outlet in a typical year. Some hydrologic datasets mark these areas separately.
• Fill-and-spill behavior: During wet periods, a pothole can fill until it overflows into a neighboring basin or channel. The area that actually contributes water downstream can therefore grow and shrink with the weather.
• Groundwater divides: The boundary that controls water moving underground does not always line up with the surface divide. A wetland can gain or lose water through the ground across a line drawn on a surface-drainage map.
• Human changes: Roads, culverts, ditches, drain tiles, dikes and water-control structures can move water across natural divides or hold it in place. Wetlands managed for wildlife often have engineered inflows and outflows that a purely topographic map will not show.

Two causes are well established: topography controls surface drainage, and glaciation shaped the pothole landscape. It is harder to say exactly how much water a particular basin contributes in a given season. That depends on precipitation, soil conditions, snowpack and local management. A boundary on a map is a model of typical surface flow, and it does not guarantee where water goes in every year.

How the US Organizes Watersheds on Maps

Most US drainage maps use the hydrologic unit system maintained through the national Watershed Boundary Dataset. Each hydrologic unit has a code, called a HUC. Longer codes mean smaller, more detailed units nested inside larger ones. Knowing the levels helps you work out what scale a map is showing.

HUC digits	Level name	What it represents
2	Region	Major drainage, such as a large river system
4	Subregion	Large portion of a region
6	Basin	Major river basin within a subregion
8	Subbasin	Medium-sized drainage, commonly used in planning
10	Watershed	Smaller drainage within a subbasin
12	Subwatershed	Local drainage area, the most detailed standard level

Montana is a good example of how large these systems can be. Most of the state, including its northeastern prairie, drains to the Missouri River and on to the Gulf of Mexico. A small area in the northwest mountains drains toward Hudson Bay, and land west of the Continental Divide drains to the Pacific. State and federal GIS portals, including Montana's state GIS data hub, distribute boundary layers like these. You can open them in free mapping software or in an online map viewer.

Delineating a Watershed on a Topographic Map

Tracing a watershed by hand on a paper or digital topographic map is a good way to understand what the boundary means. Contour lines connect points of equal elevation. Water flows downhill at right angles to those lines.

• 1. Mark the outlet. Choose the point on a stream, lake shore or wetland edge for which you want the watershed.
• 2. Find the streams. Trace the channel upstream from the outlet, along with any tributaries that feed it. On many US topographic maps, solid blue lines show perennial streams and dashed blue lines show intermittent ones.
• 3. Read the contours. Contours bend into a V or U shape where they cross a valley. The point of the V faces upstream, toward higher ground.
• 4. Locate high points. Find hilltops, saddles and ridges around the stream network. Hilltops often appear as small closed contour loops.
• 5. Draw the divide. Starting at the outlet, draw a line uphill that crosses contours at right angles and connects the high points around the drainage. Keep the line between your streams and the neighboring ones, then close it back at the outlet.
• 6. Check flow direction. Pick a few spots just inside your line and imagine water running downhill. It should flow toward your outlet. Spots just outside the line should flow away.

On flat prairie, contour intervals may be too wide to show subtle rises, and closed depressions appear as contour loops with small tick marks, called hachures, pointing inward. Where a hand-drawn divide passes through a field of these depressions, treat the line as approximate. It is better to note the uncertainty than to force a confident boundary.

Delineating a Watershed in GIS

GIS software automates the same logic using a digital elevation model (DEM). A DEM is a grid of cells, and each cell stores an elevation value. Desktop GIS packages, both free and commercial, follow a similar sequence of steps, although the tool names vary.

1. Load a DEM for the area of interest
2. Fill sinks        -> remove small pits that would trap simulated flow
3. Flow direction    -> assign each cell the downhill direction to a neighbor
4. Flow accumulation -> count how many upslope cells drain through each cell
5. Set pour point    -> place the outlet and snap it to the high-accumulation channel
6. Watershed         -> select every cell that drains to the pour point
7. Convert to polygon and compare against mapped streams and imagery

The sink-filling step needs care on glaciated prairie. Filling removes data errors, but it also erases real potholes that hold water. The result can show surface connections that rarely exist on the ground. Many analysts limit how deep a sink can be filled, or map closed depressions separately as noncontributing areas. Roads and embankments can create the opposite error, blocking modeled flow where a culvert actually carries water through. Compare the output with aerial imagery and the published hydrologic unit boundaries before relying on it.

Using Drainage Maps to Understand Prairie Bird Habitat

Watershed maps help explain why prairie wetlands differ so much from one another. That matters to anyone watching waterfowl, shorebirds, grebes or marsh birds. A wetland's watershed shows where its water comes from. A basin fed by a large upslope area may hold water longer in a dry year than a small isolated pothole. Closed basins lose water mainly by evaporation, which can concentrate dissolved salts over time and produce the alkaline or saline lakes found in parts of the northern plains. Water chemistry and depth shape which plants and invertebrates live in a wetland, and those in turn influence which birds use it.

Before a visit, a watershed map can show you how a refuge's lakes, creeks and potholes connect. It can also show which features are likely to be seasonal and where water moving through farmland or grassland reaches a wetland. Treat the map as an aid to interpretation, not as a guide to access. Drainage lines and boundaries say nothing about which land is open to visitors. Current water levels and closures should come from the managing agency.