The GCPO LCC has added a data-rich historical map of the Mississippi Alluvial Valley from Cairo, Illinois to the Gulf of Mexico to our Conservation Planning Atlas. The map, published in 1899, compiles several Corps of Engineers mapping projects dating back to 1859. The principle focus of the map is lands subject to overflow, but many other features are present, including over a dozen elevation profile transects, thousands of tributaries, and numerous named prairies in Arkansas and Louisiana. You don’t need to be a cartographer to recognize the aesthetic beauty of this well-designed and richly detailed image. Anyone who has put together a map product of any kind and who is thereby familiar with the trade-offs between presenting a high volume of information and maintaining legibility will appreciate the original, high resolution image available here.

Yvonne Allen, GCPO LCC Aquatic Habitat Analyst, stitched together and georectified the archived map images and imported the result as a dataset layer to the Conservation Planning Atlas. She added the dataset to a map and overlaid it with the Inundation Frequency Mosaic dataset she published in 2015. The Inundation Frequency Mosaic used Landsat 5 and 7 Climate Data Records from 1983 to 2011 for the entire GCPO, selected cloud-free, leaf-off conditions, and used an algorithm to classify pixels as open water, flooded land, or dry land. For each of the 50 Landsat scenes, or 185 x 185 km Landsat grid units, 15 – 40 individual images were used, representing a variety of hydrologic conditions over time. Allen generated a scaled inundation frequency index ranging from 0 – 100, with zero representing less frequent and 100 representing more frequent inundation, for each pixel based on the portion of images showing inundation at that location.

Having the two layers in the same interactive map demonstrates a striking contrast between the amount of floodplain affected by the river then and now. Most conservationists in our region are well-aware of the fact that Mississippi River is highly modified: this map comparison shows the spatial pattern of that modification. Allen (2015) notes that, although the Mississippi River has the largest drainage in North America, the current floodplain width is typically less than 15 km. Many areas in the old floodplain are still subject to frequent inundation, but only as a result of hydrological drivers unrelated to the Mississippi River.

The second notable observation is the spatial accuracy of the nineteenth century surveyors and cartographers. They knew their science and technology. Use the swipe tool to compare the 1899 map to the USGS topographic base map and notice how well many of the stream channels and town locations align.

And lastly the 1899 map shows a great number of named prairie locations west of the Mississippi River, many with intriguing names such as “Cyprès Mort Prairie,” “Prairie des Femmes,” and “Boeuf Prairie.” Archived maps such as these provide valuable information about lost features of our ecosystems and help guide our understanding of “reference conditions” for conservation efforts.

Work cited: Allen, Y. 2015. Landscape Scale Assessment of Floodplain Inundation Frequency Using Landsat Imagery. River Research and Applications 32: 1609-1620.

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