What Once Was: Wisconsin's Twenty-mile Empire Prairie

Recently I have been using the original land survey plat maps to determine in detail the size and location of the Empire Prairie. This large prairie was a vast grassland north of Lake Mendota. There are only a few prairie remnants left of this fertile land, which is now almost completely subsumed by agriculture.

There were two products of the land survey from the 1830s and 1840s. The first was the original measurement establishing each township, section lines, corner posts and quarter posts (chains and links). The second product was a detailed plat map of each township derived from the original data. The plat maps were not done by the surveyors. Rather, their records were transferred to the Surveyor General's Office in Cincinnati (later Dubuque), where skilled cartographers drew plat maps based on the original data. These plat maps were quite accurately drawn, and although they mention nothing about trees, they lay out the prairies precisely. Plat maps for three townships (Arlington, Vienna, and Westport) that contain lots of pieces of the Empire Prairie are shown below.

All of the surveyor's notes and plat maps are available online, via the Wisconsin Cartographer's Office (WCO). I downloaded the relevant plat maps and brought them into Photoshop so that the handwritten notes etc. that were outside the township boundaries could be deleted. I then brought each image into GIS (ArcMap 10.2) and georeferenced it against a township shape file obtained from WCO. The prairie boundaries on each township plat map lined up fairly accurately with the boundary in the next township.

I also used a Basemap layer obtained from ArcGIS Online, which provided township names and section numbers, as well as roads, municipalities, and water bodies.

Once all 14  townships that had parts of the Empire Prairie had been loaded, I proceeded to draw the outlines of the prairie. I created a single polygon that spanned all 14 townships.

There were a few parcels within the prairie that were oak savanna, and these had been outlined by the cartographer. They are shown on the final map as inserts.

Once the outline was completed, I estimated the percent of prairie in each township. These data are shown in the table below. (The areas outside the prairie boundaries are either bur or white oak savannas. I obtained this information from a shape file created from the original surveyors records that was kindly provided to me by David Mladenoff of the UW-Madison Forest Ecology department.)

Finally, I located existing prairie remnants and located them on the map, using the topographic Basemap and 2010 air photos for the counties of Dane and Columbia. These remnants are shown as stars on the map, and their names are given in the table.

Several conclusions can be made. The total acreage of the Empire Prairie amounts to about 150,000 acres. However, the original surveyor's records show that the vegetation surrounding the prairie was primarily oak savanna, mostly bur and white oak. Because the prairie burned annually, it seems likely that the oak savannas also burned, so that the fire swept across the countryside in a continuous line. (Early settlers have described such fires.) There are no significant water bodies to block the movement of the fire. (Tiny Goose Pond is shown on the plat map.)

How close is my estimate to the historical data. The Empire Prairie was sometimes called the "twenty-mile prairie", since it was about 20 miles across. Since each township is 6 miles on a side, it is easy to see that 20 miles is not too far off.

My map shows that the prairie runs in a southwest/northeast direction. This seems reasonable, since the prevailing winds are from the west, and Lake Mendota blocks the whole south side from fire.

The existing prairie remnants are very tiny. Some are only a few acres and the largest (Westport Drumlin) is only about 16 acres. Obviously very little prairie is left. It is impressive that much of this remnant prairie has maintained itself with very little restoration work, or even very few burns.

Arlington Township
The vertical line across the middle is the route of the Military Road (Blue Mounds to Fort Winnebago; now Portage).
Goose Pond is the feature in Section 25 (extends east into Leeds Township).

Vienna Township
Note how the Military Road line connects to that of Arlington.

Westport Township

This simplified map (GIF format) is to permit fast downloading.
A much larger version with roads, contours, and other features is available and can be provided.

Township name	Approximate percent prairie	Prairie remnants in township	Ownership
Arlington	80%
Leeds	75%	UW farm	UW-Madison
Vienna	75%	Hauser Road	TPE
Vienna		Ripp 1 & 2	Private
Windsor	70%
Bristol	50%
Dane	50%
Hampden	40%
Lowville	40%	Hagen	DNR
Springfield	40%	Koch	DNR
Springfield		Pheasant Branch	City of Middleton
Westport	40%	Bolz	UW Arboretum
Westport		Westport Drumlin	DNR
Westport		Koltes 1 & 2	NHLT
Lodi	30%
Fountain Prairie	25%
Otsego	20%
Columbus	10%

23,040 acres in a township; approximate acreage original Empire Prairie: 150,000

For a nice cultural history of the Empire Prairie, see the following publication: Martha S. Engel and Andrew W. Hopkins. 1956. The Prairie and Its People. Bulletin 520, Agricultural Experiment Station, University of Wisconsin, Madison.