<p>The causes vary around the world, but we can make some generalizations. Seasonal floods mean that there's more water coming in than going out for only <em>part</em> of the year. If it's annual, the changes are probably related to annual weather patterns in precipitation, temperature, or both. Some big sub-cases are: </p>

<p>(a) in Mediterranean climates, e.g. California, almost all the rain falls in winter in cool weather when evapotranspiration is relatively low. The Central Valley has regions that develop standing water from the rain that falls directly on them because the soil has impermeable clay or hardpan (the vernal pools prairies); and other regions that develop standing water because they're the confluence of several drainages and in wet years the water literally can't flow out to the sea as fast as it comes in (the Yolo Bypass). </p>

<p>(b) If annual precipitation includes a lot of snow and it melts in a hurry, it's common for the downstream rivers to flood at the confluences. There may be years in which the surface snow is melted while the ground is still impermeably frozen, which would turn anyplace level swampy. Parts of Minnesota have bedrock not far under the soil surface and they get this kind of inundation relatively easily. </p>

<p>(c) Where rivers meet the ocean in a long, nearly level bench, high tide (spring tide or storm tide especially) can raise the river level enough that the fields flood. I believe this is the case in the Somerset Levels and in the Netherlands. Some of these are watermeadows and can be quite productive if farmed carefully. </p>

<p>These don't all develop the same kinds of vegetation. For instance, I think I've forgotten a tropical or semitropical case where monsoon rains overwhelm the drainage (possibly the entire Amazon, where the hardwoods are adapted to having their roots underwater even though they aren't flooded all year). </p>

CC BY-SA 3.0
2015-04-29T17:43:20.883Z
Re b) in California & Northern Nevada on the other side of the mountains, consider the effect of a series of warm "Pineapple Express" storms on a substantial snowpack.  A repeating pattern, of which January 1997 was the most recent example.
jamesqf
CC BY-SA 3.0
2015-04-29T19:00:37.127Z
Good one. Also happened in western Western Washington this year -- a lot of roads way underwater.
cphlewis
CC BY-SA 3.0
2015-04-29T00:02:49.053Z
2015-04-29T00:02:49.053Z
cphlewis
<p>I am trying to determine what could cause periodic flooding of the type that might generate a savannah or plain with low growing plants and grasses. Where does the water come from, and how or why is it consistently periodic? </p>

<p>This question refers to no specific area. It is asked with a worldbuilding mindset, and I'm just trying to figure out how periodic flooding works in general. </p>

CC BY-SA 3.0
2015-04-28T20:14:17.580Z
Welcome to Earth Science! Could you be more specific about your question? Where in the world are you referring to?
arkaia
CC BY-SA 3.0
2015-04-28T20:38:06.270Z
I dont want to modify this to destory the nature of the question, but my guess is that you want to know what and why do some water sources cause periodic flooding?
Neo
CC BY-SA 3.0
2015-04-28T21:12:51.537Z
@Neo You could say that. I'm asking this with a worldbuilding mindset, but I thought it might be more appropriate here, since it is just a question about how the earth works.
Thomas Reinstate Monica Myron
CC BY-SA 3.0
2015-04-28T21:16:45.110Z
@aretxabaleta I've edited my question to make it clearer.
Thomas Reinstate Monica Myron
<flooding><world-building>
What causes periodic floods?
Thomas Reinstate Monica Myron
# What causes periodic floods?

(Asked by: username_0 on 2015-04-28)

I am trying to determine what could cause periodic flooding of the type that might generate a savannah or plain with low growing plants and grasses. Where does the water come from, and how or why is it consistently periodic?

This question refers to no specific area. It is asked with a worldbuilding mindset, and I'm just trying to figure out how periodic flooding works in general.

**Tags:** flooding, world-building

> **Comments:**
> **username_1**: Welcome to Earth Science! Could you be more specific about your question? Where in the world are you referring to?  
> **username_2**: I dont want to modify this to destory the nature of the question, but my guess is that you want to know what and why do some water sources cause periodic flooding?  
> **username_0**: @username_2 You could say that. I'm asking this with a worldbuilding mindset, but I thought it might be more appropriate here, since it is just a question about how the earth works.  
> **username_0**: @aretxabaleta I've edited my question to make it clearer.  

---

## 1 answers

### Answer by username_3 on 2015-04-29. Score: 6

The causes vary around the world, but we can make some generalizations. Seasonal floods mean that there's more water coming in than going out for only *part* of the year. If it's annual, the changes are probably related to annual weather patterns in precipitation, temperature, or both. Some big sub-cases are:

(a) in Mediterranean climates, e.g. California, almost all the rain falls in winter in cool weather when evapotranspiration is relatively low. The Central Valley has regions that develop standing water from the rain that falls directly on them because the soil has impermeable clay or hardpan (the vernal pools prairies); and other regions that develop standing water because they're the confluence of several drainages and in wet years the water literally can't flow out to the sea as fast as it comes in (the Yolo Bypass).

(b) If annual precipitation includes a lot of snow and it melts in a hurry, it's common for the downstream rivers to flood at the confluences. There may be years in which the surface snow is melted while the ground is still impermeably frozen, which would turn anyplace level swampy. Parts of Minnesota have bedrock not far under the soil surface and they get this kind of inundation relatively easily.

(c) Where rivers meet the ocean in a long, nearly level bench, high tide (spring tide or storm tide especially) can raise the river level enough that the fields flood. I believe this is the case in the Somerset Levels and in the Netherlands. Some of these are watermeadows and can be quite productive if farmed carefully.

These don't all develop the same kinds of vegetation. For instance, I think I've forgotten a tropical or semitropical case where monsoon rains overwhelm the drainage (possibly the entire Amazon, where the hardwoods are adapted to having their roots underwater even though they aren't flooded all year).

> **Comments:**
> **username_4**: Re b) in California & Northern Nevada on the other side of the mountains, consider the effect of a series of warm "Pineapple Express" storms on a substantial snowpack.  A repeating pattern, of which January 1997 was the most recent example.  
> **username_3**: Good one. Also happened in western Western Washington this year -- a lot of roads way underwater.  

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