4 replies to this topic

[JohnD]

I just got back from a three-day trip to Taitung and Hualien, and along the way we crossed a lot of rivers. Almost every one of them was mostly just dry riverbed (gravel and sand/dirt) with just a small stream cutting its way through what was once obviously a much wider river. Why are so many of these rivers dried out like this? Does anyone know?

[Jaak]

I just got back from a three-day trip to Taitung and Hualien, and along the way we crossed a lot of rivers. Almost every one of them was mostly just dry riverbed (gravel and sand/dirt) with just a small stream cutting its way through what was once obviously a much wider river. Why are so many of these rivers dried out like this? Does anyone know?

My informed guess:

The mountains of Taiwan are pretty steep, and from what I remember, they consist of impermeable rock mostly, not rocks like limestone that could hold large amounts of groundwater.

When a typhoon or a thunderstorm comes in the mountains, the water runs straight down the steep slopes and through the riverbeds, washing gravel and sand down from the mountainslopes. And when the rain stops, the water flows into the sea in a few days and then there is only a little water left trickling out of springs.

Fast flowing floodwaters laden with gravel and sand easily scour young plants away. It is enough if the river floods just a couple of times in summer, and the bed will stay bare for months afterward, with only a small trickle in the bottom.

Can anyone confirm or correct me?

[shunyadragon]

My informed guess:

The mountains of Taiwan are pretty steep, and from what I remember, they consist of impermeable rock mostly, not rocks like limestone that could hold large amounts of groundwater.

When a typhoon or a thunderstorm comes in the mountains, the water runs straight down the steep slopes and through the riverbeds, washing gravel and sand down from the mountainslopes. And when the rain stops, the water flows into the sea in a few days and then there is only a little water left trickling out of springs.

Fast flowing floodwaters laden with gravel and sand easily scour young plants away. It is enough if the river floods just a couple of times in summer, and the bed will stay bare for months afterward, with only a small trickle in the bottom.

Can anyone confirm or correct me?

Yes, this the nature of Taiwan hydrology, but Taiwan also has problems of drought, particularly since 2002. It is worse in the south, and partly due to global climate change.

[RollingWave]

Yes, that is correct, the combination of steep mountains and short river course and great variation of rains in different season causes this, IIRC only the River in the Taipei does not suffer from this .

You would noitce that most of those rivers have a very large strech of uninhabited space along side the river right? and the dikes to prevent flood is built wayyyyyy beyond where the little stream is, that's because when a typhoon hits that's how big the river becomes.

[JohnD]

You would noitce that most of those rivers have a very large strech of uninhabited space along side the river right? and the dikes to prevent flood is built wayyyyyy beyond where the little stream is, that's because when a typhoon hits that's how big the river becomes.

That's right. It's like a big gravel field. You can clearly see how wide the rivers become at times.

Thanks Jaak and RollingWave for the explanation.