Rivers II: Big Fun on the Bayou

Not only are swamps thematically interesting, wetlands are important sources of early iron (and peat or coal). So it's useful to know where they might form. This is not as easy as it sounds - although there are reams of scientific literature on wetlands, they almost always focus on description rather than prediction (because we observe wetlands after they've formed), and even those definitions can be fuzzy.

There are four overarching types:

• Swamps. They are forested, and the other types are not. They often form on the banks of large rivers or lakes.
• Bogs. These form in the highlands, with a high precipitation which exceeds the evapotranspiration (generally this is the same as PET, or at least for my purposes). Bogs are the best source of bog iron, as well as peat, bog oak, and many types of berries. Occasionally they can form in glacial valleys or at high headwaters. Bogs specifically show up in cold temperature and boreal climates.
• Marshes. These form on the coasts of seas, lakes, and streams in temperature or high latitudes.
• Fens. These form on slopes, plains, or depressions (as opposed to bogs). Both bogs and fens are mires (containing peat), but bogs are acidic and fens are alkaline.

From this, I need to establish selection rules. In each case below, I'll select a random number of the locations which meet the criteria. In addition, the aridity ratio ($P/PET$) must be above 1.

• If the Holdridge type includes forest, and a river with a drainage above a certain threshold flows through the hex, and the hex is a plain surrounded by at least 3 other plains, the type is swamp.
• If the Holdridge type does not include forest, and a river with a drainage above a certain threshold flows through the hex, and the hex is a plain surrounded by at least 3 other plains, the type is marsh.
• If a coastal hex has an elevation lower than 50 feet, and is a plain, it is a marsh.
• If a hex is a highland or valley, and the annual precipitation is higher than a threshold, it is a bog.
• If the hex is a depression, and the annual precipitation is higher than a threshold, it is a fen.
• If there is a headwater in the hex (defined as the absence of any other hexes which drain into it) and the drainage is higher than some threshold (indicating a high amount of rainfall), and the hex is a plain, it is a bog.

I may revisit the bog/fen distinction if I ever dip into soil determination.

These are some pretty strict requirements. However, fiddling with knobs like this is the best way to get a world map with reasonable variety.

This classification scheme is not perfect but my hope is that it has the ghost of scientific backing, and provides enough distinction that I can convey to my players the difference between the bayous of Louisiana to the bogs of Siberia.