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8 May, 2008

 Products The Guns of Gettysburg Design Diary Unbottled Water

The Guns of Gettysburg Design Diary: Unbottled Water

Having dealt with relief in the last entry, this entry will take up one of the major forces of nature that creates relief: water. From a research point of view, Gettysburg poses no particular challenges, but hydrology can be a problem in map development. While relief generally changes only slowly, hydrology is different; both natural and man-made events can produce very substantial changes in short periods of time, which has the potential to greatly complicate battlefield reconstruction.

Natural changes primarily affect the courses of streams and rivers. While rivers running through rocky or hilly ground are relatively stable in their courses, when the ground is flat and consists mainly of silt deposited by floods, major (and quite sudden) course changes are possible. A striking example of this can be seen by looking at the course of the Bormida River near the battle of Marengo. Below to the left you can see the course of the river at the time of the battle, and to the right its course eighty years later.

The Bormida River, 1800
The Bormida River, 1880

In seeing major discrepancies between old and new source maps like this, it is worth considering whether the old map might simply be wrong. One quick test for the old map is to look at its general reliability: does it generally match up where it should? In the case of the 1800 Marengo map, we can see that it does; the course of the Fontanone, for example, which is a small stream that roughly parallels the Bormida to the east, is almost identical in the 1800 and 1880 maps. It just doesn’t seem possible that if the 1800 map did such an outstanding job of depicting the course of a small stream like the Fontanone (and the map is clearly accurate for scores of other minor terrain features as well), that it could have made such a gross error regarding the course of a major river.

While such large changes can at first surprise, how they can take place is actually reasonably well-understood: when a bend starts in a river, the water on the outside of the bend goes faster and carries away more of the bank while the water on the inside of the bend goes slower, and deposits silt. Thus, over time, the bends get bigger and bigger and a narrow neck forms; this neck can close either because of a flood or the collapse of the banks and when it does the river ceases to flow into the meander, which sometimes may be left dry and sometimes may form an oxbow lake (the 1800 map shows such an oxbow lake just north of the center of the frame). Additional information on how rivers change their courses over time can be found here.

Nature can certainly make life difficult for hydrologic research, but sometimes man can be at least as troublesome. In the battle of Austerlitz, there were some shallow ponds towards the southwestern end of the battlefield that had a substantial effect on the course of the battle. Older maps, however, do not agree on the size or shape of the ponds, and because they were drained some years after the battle, on more modern maps they don’t exist. There was also no consensus, as far as I was able to see, among modern historians on the subject, who also showed them as having a variety of shapes and sizes. Sometimes, however, more modern maps can still show traces of old hydrology that can be very helpful in reconstructing it. In the Marengo maps above, for example, the oxbow lake referred to on the 1800 map may have dried up by 1880, but some traces of its past presence are still visible on that map. Similary, for Austerlitz, the vanished ponds may have been drained prior to the creation of the 1:75,000 Austro-Hungarian 1878 military map that was one of my main sources on Austerlitz, but traces of them still existed, allowing me to verify that the shape shown on the Austerltiz map that accompanied Alison’s History of Europe (another map source I used) was at least broadly accurate, and even suggested possible distortions of how the vanished ponds were depicted in the older map:

Kobelnitz Pond, Alison’s Atlas Map
Kobelnitz Pond Remnants, 1878 map
Satschan Pond, Alison’s Atlas Map
Satschan Pond Remnants, 1878 map

Gettysburg posed no such interesting research challenges when it came to hydrology. While it is true that modern maps such as the 1999 1:25,000 USGS map show many artificial lakes and ponds that weren’t there at the time of the battle, their absence on the 1946 and prior maps made it easy to confirm that they were post-battle features. Most of these were quite small (a hundred yards across or so), and you can see seven of them (by my count) in this one segment of the battlefield around the mother of all Gettysburg battlefield artificial bodies of water, Lake Heritage:

Future Site of Lake Heritage, 1946
Lake Heritage, 1999

From a map art perspective, I’ve never done much with streams, creeks, runs, rivers, etc. (you know what I mean, those things where the water flows downhill). The easiest way to depict streams is just with a uniform-length blue line that follows the course of the stream, and for small streams, it generally doesn’t make much sense to attempt more than that as any fine detail is too small to see anyway. For rivers and larger streams, a more detailed approach is appropriate: actually tracing the shoreline so as to re-create (or create the appearance of re-creating since you don't always in fact know that level of detail) the river. Some examples are shown below:

Small Stream
Uniform Width
Larger Stream
Variable Width

As it happened, Gettysburg had a number of small marshy areas. They didn’t play any significant role in the battle and really we probably wouldn't even know they were there except that the 1868 map was on such a large scale and so carefully done. None of the later maps show them, and I saw only ambiguous evidence of them at best in aerial imagery. Here is the largest of the swamps across the usual suspect list of map sources (incidentally the changing length of the stream is unlikely to reflect any real changes in the stream; there is a lot of map to map variation as to the sources of streams due to differences between particular cartographers in deciding when a stream is just too small to bother with):

Swamp in 1868 Map
Same Area in 1904 Map
Same Area in 1946 Map
Same Area Today

Swamps are traditionally shown on maps using a group of symbols, what appear to be clumps of long grass, sometimes with and sometimes without a horizontal line underneath to indicate water. In Bonaparte at Marengo and Napoleon’s Triumph I went with the same no-line symbol for both games. The Guns of Gettysburg uses a with-line symbol and gets a small visual upgrade in that rather than using exactly the same symbol over and over, it uses a few different symbols. I can’t say I think it really makes much difference, but I thought it worth a try.

Swamp in Napoleon’s Triumph
NT Swamp
Swamp in The Guns of Gettysburg
GoG Swamp

Anyway, it is time, I suppose, to show you the Guns of Gettysburg map with the hydrology layer turned on. You may notice a color shift from the previous diary entry and may see more in the future as I play around with color. As before, click to view a detailed map in its own window:

The Guns of Gettysburg Hydrology Map

Gettysburg Hydrology

Click on the image above to open in its own window