The Rosgen system breaks rivers down into one of nine types, based on a number of physical characteristics. Importantly, for our purposes, one of those characteristics is slope. Also importantly, for our ultimate purpose of working out the flow velocity of the river, one of the characteristics is width-to-depth ratio. In this post, I'm going to show how to use the slope of the river to determine the Rosgen type, and then use that figure out a width-to-depth ratio (from here on out referred to as WDR) so we can plug it all into Manning's formula. Also, since it has immediate bearing here, we'll look at another river characteristic, sinuosity, and how it affects slope.
Okay, a quick overview of the types of rivers...first up is type Aa+. Any river with slope of more than 0.10 will be Aa+. Note that you won't randomly determine any rivers this steep using the system I supplied earlier; they just don't run that steep for the distances most hex maps cover. Rivers and streams this steep are going to be a lot smaller than a 20 mile hex, but almost any mountain or hill (of which there are usually quite a few in a 20 mile hex) will have one or more draining off of it, but the stream bed will flatten out as it reaches the bottom and become another stream type. Aa+ rivers tend to erode their beds until solid rock is reached, carrying away the sediment to be deposited downstream. They tend to include waterfalls or cascades along their length (the Dungeoneer's Survival Guide gives a guideline of 3d6 drops of d8 feet each per mile for streams this steep, but steeper drops can be placed as desired).
Any river with a slope of 0.04 to 0.099 is going to be type A. Type A is a lot like type Aa+, just not as steep. There's probably a lot of technical details to separate the two, but for our purposes, I'm going to say they might as well be the same, but slightly more sinuous, and more likely to show up in lengths long enough to appear on a large area hex map. If you look at the map of Mystara, the Streel River runs across most of the continent, and even at a gentle slope, the headwaters in the far west of Vestland end up at pretty high elevations, meaning the rivers draining eastwards through Vestland have a long way down in just a few hexes travel; they're almost all going to be type A rivers.
Type B rivers have slopes of 0.02 to 0.039. They tend to have a lot of rapids, and as such, can use the Dungeoneer's Survival Guide guideline of 0-5 drops of d4 feet per mile, although at the higher end they can use the cascades guidelines above, and the lower end of the slope range would be closer to the medium grade guideline of a 1/3 chance for a drop of d3 feet per mile.
Type G rivers fall into the same slope range as type B, but they are more entrenched and narrower (lower WDR), taking on a "gully" form. They typically arise in areas with unstable banks, or as a result of ill-advised modifications to other river types (straightening out meanders, dredging an otherwise stable bottom, etc).
Type D rivers can occur at any slope less than 0.04, typically arising when a fast flowing, sediment laden river suddenly slows down, usually because of a flattening in the slope. The river drops most of the sediment it was carrying, resulting in a wide "river" made up of multiple smaller channels running through shifting bars of gravel or sand. The listed WDR for type D rivers is for the whole system. Glaciers often form type D rivers where the sediments dredged up and trapped by the glacier are dropped by the melting ice.
Type C rivers occur at gentler slopes (less than 0.02), and are meandering rivers with broad floodplains. A defining factor in type C rivers is the existence of point bars, sandy shallows on the inside curve of each meander, with the deeper main channel pushed toward the outside of the curve.
Type E rivers are found at the same slopes as type Cs, but tend to be much deeper in relation to their width (lower WDR). Type E rivers have very stable, vegetated banks.
Type F rivers are also found at slopes less than 0.02. They tend to occur in easily erodible soils, and as such differ from type C and E in that they are more entrenched into the surface of the earth, and have unstable banks.
Lastly we have type DA rivers, found at slopes less than 0.005. Like type D rivers, they are wide affairs made up of multiple smaller channels, however these channels don't shift like those of type D rivers, the banks and bars being anchored by heavy vegetation. It's probably a safe bet that any swampy areas on the map are composed of type DA rivers, even if no river icon is noted on the map. Any river running through a swampy area can be assumed to be type DA, and river deltas tend to be type DA as well.
So, from these descriptions, some river hexes on any map are already sorted into a particular type, either from the slope or from what we know about the surrounding terrain. If two or more river types can conceivably work in a given hex, the DM can decide which one to use via random die roll.
One last thing before ending this post, because it affects the final effective slope of the riverbed: each river type has a range of sinuosity scores associated with it. What this refers to is the amount of wiggle there is in the river. If a hex is 20 miles across, but the river zigs and zags within that hex before flowing out, a boat traveling along it might cover 40 miles or more of distance in that space. If that the river drops 500 feet moving along that hex, the slope of the river should be (500 ft/40 miles) = 0.002 rather than (500 ft/20 miles) = 0.004. To represent this, divide the preliminary slope value found earlier by the sinuosity score to find the effective river slope.
Here is a handy chart (based on the earlier elevation chart, and which I will probably expand on in later posts as I go into more detail about river types) showing the river types grouped by slope values, and with width-to-depth ratios and sinuosity scores listed below.
|1||0.0001||x||x||x||x||x||placed as needed|
|Sinuosity||1||Min 1.4*||Min 1.5*||Min 1.4*||1||Min1.2*||Min 1.2*||1.0 – 1.2||1.0 – 1.1|
* For these river types, roll 1-6 d6, divide by 10, and add 1, for a total of 1.1 to 4.6. If this result falls under the minimum listed, use the minimum instead.