OGF:Making realistic rivers

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This page is designed to help users create rivers that are realistic.

The easiest way to produce realistic rivers is to imagine your topography first...
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...and then, imagine how the water would flow down the mountains and valleys you've created:
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The pattern of rivers can be very different, depending on how you've arranged your mountains and valleys:
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But in all cases, the water flows from higher ground to lower ground.

And rivers combine as they flow downward.
So, a few pointers:
1. Don't let rivers split as they flow downhill. There are a tiny handful of places in the real world where this actually happens, but it is extremely rare (like, one-in-a-million rare). Instead, rivers tend to combine as they flow downhill. The only situations where rivers commonly split is to form a small river island or a wide flat delta. Otherwise, splitting virtually never happens.
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2. Don't let one river branch off to join another. Same principle as above.
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3. River islands should be relatively small. Again, same principle as above. River islands can actually get quite large in a large delta, but inland river islands tend to be fairly small.
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4. Don't let a lake have more than one outflow. Many rivers can flow into a lake, but the only outflow will occur at the lowest point along its rim. Like the splitting rivers phenomenon, there are actually a small handful of exceptions in the real world, but they are extremely rare (one-in-a-million).
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5. Don't let a river flow from the ocean back to the ocean. Rivers cannot flow uphill onto land and then back downhill into the ocean.
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6. Don't let a river flow from one ocean into another ocean. Same principle as above, but it can be extra tempting to connect two different oceans. Instead of using a river, use a canal for this (if a canal is economically feasible).
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7. Rivers tend to follow the quickest route to the sea. It can look awfully fishy if you make a river avoid a more direct route in favor a less direct route. There are some places in the real world where stuff like this happens, but it's quite rare. Be careful.
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8. Don't let rivers follow the centers of islands or peninsulas. Drawing a river along the middle of a peninsula or across the center of an island can seem intuitive for some reason, but it doesn't happen much in real life and can definitely look weird.
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9. Be careful about doing crazy things. Wild and wacky river systems do happen sometimes in the real world, but be careful! If they're not carefully done, they can easily make things look confusing and unnatural. When in doubt, simplicity is a good strategy.
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10. Make river curves the right size. Everyone knows that rivers curve, but most people don't know that the curves tend to follow certain rules. One common mistake is making the curves too big, for example:
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There are many different types of rivers, but notice that none of them have enormous round curves:

A - Straight river
B - Braided river
C - Mildly meandering river
D - Heavily meandering river
E - Anastomosing river

This is because the ratio of a river's width to its curve almost always follows a certain mathematical pattern:

width : curve radius : meander length
1 : 2.3 : 11

This tends to hold true for any smoothly round river curves, no matter how wide the river is.

On the outer edge of the curve, the water is deeper, as the river works away at the land, sometimes even forming a cliff along the bank. On the inner edge of the curve, the water is shallower and calmer, with maybe some wetlands, wet forests, or little lagoons from old loops of the river.
11. Make rivers flow in the right direction. On the OpenGeofiction map, they should be drawn so that the direction of the way represents the flow of the water:
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12. Make rivers detailed. Instead of merely representing rivers with lines, use the "waterway=riverbank" tag to create additional polygons that illustrate the water area of the river. This makes your mapping look a lot more interesting and realistic. Be sure to show river width using the "waterway=riverbank" tag, instead of using long indentations in the coastline. For anyone who is interested in calculating how wide a river should be, see the section below.
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How wide to make a river?

How wide is a river? The biggest factors are the rain that falls on the land, and the area (km²) of land that the river drains into the sea.

So for example, every year in central Europe 600 million m³ rain falls on 1000 km² (annual precipitation = 60 cm). Normally about 25% of the rain goes directly into the river and is carried to the sea. A little more water (about 7%) flows into the river from the groundwater. So for example a river in the village where I (User:Histor) live, carries over 170 million m³ of water in the year, draining 1000 km². Flowing 1 m per second (going down 1 m every 2 km), this river is 1.10 m deep and is 12 m wide. I have measured this ten years. Every year, about 152 to 196 million m³ of water flows past.

So you can calculate:

  • R = the rainfall in the area drained by the river, in terms of millions of m³ per year per 1000 km². In this case = 600. In a very wet country, you could use 1200, or in a dry country, maybe 300 is realistic. Higher mountains tend to have more rainfall on the side where the wind most often hits them.
  • A = the area drained by the river in 1000 km² - in this case = 1
  • B = normally the width of the river in m, if it is 1 m deep

So our first simple (and rough) formula is (R/4) * A * 0.1 = B - or in numbers (600/4) * 1 * 0.1 (means 10 %) = 15 m

Other examples:

  • 2) R = 800, K = 10 (10,000 km²); B = 200 m
  • 3) R = 500, K = 20 (20,000 km²); B = 250 m
  • 4) R = 800, K = 50 (50,000 km²); B = 1000 m
  • 5) R = 900, K = 100 (100,000 km²); B = 2250 m

Of course there's room for variation. Your river can perhaps be 30% or so wider or narrower than the calculation. And yes, larger rivers are normally deeper than 1 m... so for larger rivers it's also good to calculate the depth of the river. More water means deeper rivers. But how deep depends on complex factors, like the details of the riverbed, the surrounding topography, and the influence of humans living along the river (dredging, etc). Logically, of course, a river 2 m deep will be half as wide as a river 1 m deep, if they have the same volume of flow. The "depth"-Factor "F" you can calculate freely, but realistically it may go something like this:

  • if B = 20 - 100, then F = 2
  • if B = 100 - 200, then F = 3 - so example 2) and 3) become 100 m and 125 m wide instead of 200 m or 250 m
  • if B = 200 - 500, then F = 4
  • if B = < 500, then F = 5 - so example 4) and 5) become 200 m and 450 m wide

So the second simple formula is B / F = X. X is how wide the river really is. If the river runs through a gorge, for example, or if there are other influencing factors, it can be wider or narrower.

Where the river meets the sea

Where tides are flowing, a river is really only a long bay of the sea. So this "river" can be as wide as you like. You can mark the tidal land in your map with "natural = mud" and along the seaside of your coastline (the coastline way representing the water level at high tide).

Some rivers have deltas, the shape of which vary depending on the relative strength of the river flow, the wave action, and the tides. Here are some common delta configurations:


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