I was going to start on ecology, but there's an intermediary step between the "I've set up the planet to how I think it needs to be" and "I want to populate the planet." That is to create the geography of the area. I'm simplifying things immensely, but that boils down to mountains and rivers. Or, more broadly, determining what the elevation is in the area, and where moisture and bodies of water are. This drives so much down the road, it's important to get this right.

Mountains

First, let me tell you what doesn't exist. Picture a near-endless plain, possibly wheat or grasslands. In the distance, a single mountain juts up from the horizon, impossibly tall and snow-capped. The castle, cave mouth to a dragon's den, or Eye of Sauron at the peak is optional. While this is a beacon for adventurers that "monsters be here," this does not happen without human interference.

This is because mountains are created in one of three basic ways. Note that this is incredibly simplified version of orogeny. The first and most common way is when two tectonic plates collide. Like the Juggernaut running into the Blob, something needs to give. One plate folds up and the second plate slides under it. This causes a large mountain range. Traditionally, this causes a series of peaks and valleys.

The second way is "block" formation. Instead of two plates going head-to-head, they rub side by side or get pulled apart. This causes the plates to split apart and fracture into blocks. One of these blocks collapses and is lowered relative to the rest of the plates, or a block is raised as it slips to the side. These faults can extend thousands of miles, and while this doesn't mean the entire mountain range extends across the entire fault line, this can cause interesting results. Refer to the Great Rift Valley for more information. You have a large, mostly-level area that extends for miles, then a drop and level area. Then a raised area, then drop, then repeat. In this example, some of the drops is below sea level, which causes the Red Sea and Gulf of Aden. Cool stuff.

The third example is volcanic eruption and localized pressure. Ash or lava is pumped into the crust and settles around the fault. Because these faults are usually in a line, these volcanoes are grouped together, usually. The idea of a single mountain from the plains is most likely created from this method. If someone punched a hole into the crust, this would produce the desired outcome. And really, "Summon Volcano" would be an awesome if not suicidal spell.

In short, mountain ranges are actual ranges. They tend to be built in parent-child relationships, where there's a main trunk, and smaller branches from these. The initial size of the mountains are going to be dependent on the age and activity of the volcano, or the power of the plates that are involved in the mountains. Then, the other thing to look at is erosion. If there are a lot of forces causing erosion (wind, water), then the mountains are going to be smaller. Hills, even. Obviously, the larger branches are going to have the higher mountains, barring major tectonic activity.

The other fun thing to play with, geography-wise, is high mountains in an ocean. Hawaii is a great example of a volcano that creates a land mass. If your planet is more volcanically active, you can have dozens of these islands dotting your world. It would be interesting to see how this influences culture if every tribe had its own tropical island.

Rivers

Rivers are a little bit easier to plot. There are three parts to a river: the source (where the river originates), the mouth (where the river meets the ocean, sea, or lake), and the course (everything else in between). Under most circumstances, the river will follow a single course, but some rivers will bifurcate, or split into two. These may join later downstream, or they may separate permanently, leading to two different mouths. There are also braided rivers, or rivers that separate and combine and separate and combine. Rivers will not take the most direct path; it can meander based on gravity and rock.

The river's source is defined as the most distant point from the mouth of the river. In reality, a river may have its source be a lake, another river, or some other freshwater body. Alternatively, the source may be a freshwater marsh, headwater from a glacier or ice flow, or an underground source.

The most important thing is that rivers flow downstream, from a higher elevation to a lower elevation. If the river source is in the mountains, this is easy. Trace the route to the nearest body of water. As the river flows, it will cut a path through the mountain range, creating a gorge in the mountains. You can use the Colorado River as an example. If the river starts at a lower elevation, then it will continue to a body of water, but the area around it may flood. The width of the river will vary based on the season. Some rivers (ephemeral rivers) dry up completely, depending on the temperature and humidity in the area. Likewise, the flow of the river (which will determine how much it cuts through the environment) depends how wide the river is and how high its banks are. A wide river that is in a flood plain, like the Mississippi river, flows slowly.

The mouth of the river may end up in a delta, or a landmass that juts from the river mouth. Sediment is deposited at the river mouth, and that builds up over time. These tend to be wetlands, close to the sea, and lush. Likewise, a bend in the river will deposit sediment to the near side and erode the far bank, causing an inflated bend in the river. This may go to the point where it collapses in on itself. This, in a word, is cool.

And then there are artificial changes to the river. Dams and canals can cause rivers to change course. High technology or high magic will accelerate this to make things convenient for people, either in terms of trade and transportation, or energy production, or reduced flooding.

Why?

There's several reasons why this is important:

• Tectonic Plates: If you are doing a global map, it's important to know where the tectonic plates are. These will give you a good feel of where your land masses are. Quick trivia: did you know that the continent of Australia is larger than the country Australia? The continent includes a lot of neighboring islands, and those islands bump up the area by a half million square miles.
• Ecosystem: Two of the important components of an ecosystem, what defines an ecosystem, is temperature and humidity. Mountains and rivers directly influence both of these.
• Weather: Mountains and rivers influence weather patterns. So, if the mountains and rivers are set, you will get a better handle on what the typical weather.
• Borders: Mountain ranges and rivers create a nice border betaween areas. These borders are both geographic (for nations), and natural (for animals and plants). Animals may evolve very differently if there is a mountain range between them. Cities may become more isolated if people have to hike over mountains, or more cosmopolitan if a nearby river gives travelers easy access.

Now that you have a general sense of mountains, elevation, terrain, and water flow, the next step is to determine the ecosystems. That is next month's column. And I want to end with my shameless plug of my story, Virtual Campus. Until next month.