As with most conplanet creation, what you will do here depends on the degree of realism you want. This page assumes that you are going for a high level of realism; diversions from this are at your own discretion.
Oceans and Continents
On the Earth, the only real example we have of large bodies of water, ocean formation is tied closely with volcanism and tectonics. It appears that oceans are largely formed through water vapour expelled by volcanoes, as well as possibly from infalling comets.
The exact shapes of the continents are seemingly random, but there do seem to be tendencies:
- Islands usually form over weak spots in tectonic plates (Hawai'i) or where continents have been torn, like taffy, leaving 'bits' behind.
- If the coastline is close to a source of glaciers, it will have a jagged coastline or a rocky beach.
- Sand that flows off of a coast could create shifting sandbars and sand islands, especially if it's in a sound.
- Parts of continental plates are under water, causing relatively shallow shelf seas.
Prevailing Currents and Winds
Having a good understanding of prevailing winds will also help you to model your planet's weather.
Mountains form on converging plate boundaries. Coastal Ranges are created by subducting oceanic plates thrusting up more magma below the crust. Volcanic peaks are formed by diverging or converging boundaries where magma is channelled through the crust. Age of mountains will have an effect on their appearance. For example: Sharp, angular summits will not exist in an ancient, eroded mountain chain, but will form in a newer, more active converging boundary.
Notable characteristics about rivers:
- Rivers usually rise in mountains and then flow to the sea.
- They always start in high elevations and move to lower elevations.
- Rivers primarily flow from the seaward sides of mountain ranges to the ocean.
That is why several important rivers start in the Himalayas and flow through India to the Pacific (the nearest ocean) rather than flowing through central Asia to the Arctic Ocean. (This is not exclusively true, though; the Mississippi, for example, starts from a rather nondescript lake in Minnesota and flows to the Gulf of Mexico, all the while going through relatively dry land. However, the Mississippi still travels downslope, and does not go to the relatively more distant Hudson Bay.)
Common mistakes of conworld creators in placing realistic rivers are:
- Major rivers splitting far from their mouths. Real rivers do not do this.
- Rivers going one side of a mountain range, through the mountains, to the other side. This would require rivers flowing from a low place to a high place. Going through pre-existing broad valleys is possible, but otherwise, the river would simply flow around the mountains.
- Rivers that start in the ocean. This does not happen in real life.
- Rivers flowing two ways out of the same lake. Again, this does not happen in real life. (Except for Gatun Lake and Balsam Lake, but both lakes exist as part of canal systems).
That the height of certain geographical features varies widely from those of others seems self-explanatory, but it is a feature of Congeography that many conworlders forget to include. Knowing the elevation of certain areas on a given conmap can help with mapping rivers (as mentioned above), modeling a place's weather, and can even indirectly affect the culture of that area (due to advantages or disadvantages in location).