# Bridge on the River Kwai. Why does the water go down? **URL:** \ **Category:** Cafe Society\ **Created:** [March 3, 2008, 7:40pm UTC](https://boards.straightdope.com/t/bridge-on-the-river-kwai-why-does-the-water-go-down/439951 "2008-03-03T19:40:01Z")\ **Posts on this page:** 16\ **Page:** 1
**Author:** ![Paintcharge](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/paintcharge/32/7946_2.png) [@Paintcharge](https://boards.straightdope.com/u/Paintcharge)\ **Post date:** [March 3, 2008, 7:40pm UTC](https://boards.straightdope.com/t/bridge-on-the-river-kwai-why-does-the-water-go-down/439951/1 "2008-03-03T19:40:01Z")
Hmm… Posted this question this morning, but it seems to have disappeared. Mods, if that is a problem, please delete. Anyway, Bridge was on last night and the question of how does a river go down several, 5, maybe 6, feet overnight. I’ve lived near a few different rivers in my life, granted none of them were in a tropical rain forest, and have never seen a river that wasn’t either dammed or tidal go down that fast. I’ve seen them rise quickly during spring snow melt, but not go down. Any brilliant ideas from the teeming millions? ---
**Author:** ![JohnT](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnt/32/15048_2.png) [@JohnT](https://boards.straightdope.com/u/JohnT)\ **Post date:** [March 3, 2008, 7:46pm UTC](https://boards.straightdope.com/t/bridge-on-the-river-kwai-why-does-the-water-go-down/439951/2 "2008-03-03T19:46:53Z")
As for the movie, the river was dammed by the production crew which caused the water level to drop. [http://www.lankalibrary.com/phpBB/viewtopic.php?t=1076](http://www.lankalibrary.com/phpBB/viewtopic.php?t=1076) As far as a real-world explanation goes… I got nuthin’. ---
**Author:** ![Defensive\_Indifference](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/defensive_indifference/32/6502_2.png) [@Defensive\_Indifference](https://boards.straightdope.com/u/Defensive_Indifference)\ **Post date:** [March 3, 2008, 8:16pm UTC](https://boards.straightdope.com/t/bridge-on-the-river-kwai-why-does-the-water-go-down/439951/3 "2008-03-03T20:16:59Z")
[QUOTE=Paintcharge] lived near a few different rivers in my life, granted none of them were in a tropical rain forest, and have never seen a river that wasn’t either dammed or tidal go down that fast. [/QUOTE] I assumed it was tidal. Dunno much about tidal rivers, though. Is a tidal river possible in the location they were suposed to be in? ---
**Author:** ![Quartz](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/quartz/32/267_2.png) [@Quartz](https://boards.straightdope.com/u/Quartz)\ **Post date:** [March 3, 2008, 8:26pm UTC](https://boards.straightdope.com/t/bridge-on-the-river-kwai-why-does-the-water-go-down/439951/4 "2008-03-03T20:26:39Z")
Generally, the depth of a non-tidal river is determined by water input - icemelt or rainfall - upstream. No rainfall means a lower river. There can be significant changes in a very short time. Just ask a flood victim. ---
**Author:** ![Paintcharge](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/paintcharge/32/7946_2.png) [@Paintcharge](https://boards.straightdope.com/u/Paintcharge)\ **Post date:** [March 3, 2008, 8:47pm UTC](https://boards.straightdope.com/t/bridge-on-the-river-kwai-why-does-the-water-go-down/439951/5 "2008-03-03T20:47:07Z")
[QUOTE=Quartz] Generally, the depth of a non-tidal river is determined by water input - icemelt or rainfall - upstream. No rainfall means a lower river. There can be significant changes in a very short time. Just ask a flood victim. [/QUOTE] Agreed. I’ve seen the water in the back yard of my parent’s house go **up** 6 or more feet overnight, but never down that much. At the end of a flood the water must go downstream and the stream bed hasn’t gotten any larger. Kind of like you can only send so much through a hose at one time. I suppose the river could have been at the end of a flood stage when they arrived at night, but it had seemed pretty stable throughout the movie. Swimming soldiers etc. No big deal, just curious if anyone had an explanation for that anomalous aqueous behaviour. ---
**Author:** ![KGS](https://avatars.discourse-cdn.com/v4/letter/k/90ced4/32.png) [@KGS](https://boards.straightdope.com/u/KGS)\ **Post date:** [March 3, 2008, 8:50pm UTC](https://boards.straightdope.com/t/bridge-on-the-river-kwai-why-does-the-water-go-down/439951/6 "2008-03-03T20:50:25Z")
Plot complication? ---
**Author:** ![Hypno-Toad](https://avatars.discourse-cdn.com/v4/letter/h/b2d939/32.png) [@Hypno-Toad](https://boards.straightdope.com/u/Hypno-Toad)\ **Post date:** [March 3, 2008, 9:23pm UTC](https://boards.straightdope.com/t/bridge-on-the-river-kwai-why-does-the-water-go-down/439951/7 "2008-03-03T21:23:23Z")
The Potomac River near my old home is considered seriously dangerous because it can rise extremely fast due to upstream rain storms dumping a lot of water into it all at once. But I have no idea if it can drop quickly. But it doesn’t necessarily mean that the Kwai dropped quickly. It was significantly lower by the next day. But that was undoubtedly hours after the time the commados set the charges. So yeah, the river could have been in flood at the time they set to work and back to lower levels when the train was due. ---
**Author:** ![lieu](https://avatars.discourse-cdn.com/v4/letter/l/c2a13f/32.png) [@lieu](https://boards.straightdope.com/u/lieu)\ **Post date:** [March 3, 2008, 9:46pm UTC](https://boards.straightdope.com/t/bridge-on-the-river-kwai-why-does-the-water-go-down/439951/8 "2008-03-03T21:46:57Z")
[This](http://www.lankalibrary.com/phpBB/viewtopic.php?t=1076) site explains how they did it for the movie, but doesn’t answer your question as to how it would have occurred naturally. _“We walk downstream to the point overlooking the sand bar on the opposite bank where the detonator was set behind a large boulder. I’ve decided I must have been a dyed-in-the-wool movie addict all along. I can feel a tingle of excitement as I stand at the very place where Shears (William Holden) shouts out “kill him” to the man on the plunger as he struggles with Colonel Nicholson. What isn’t so noticeable in the film is that just downstream from here is a dam - now just a pile of rocks on each river bank - used by the movie-makers to control the river, and produce the drop in water level which reveals the explosives on the bridge and the wire which leads to the plunger.”_ The only angle I could originally come up with was a bombed dam, but they obviously were too far from such, being quite isolated judging by the conditions suggested in the film. ---
**Author:** ![panache45](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/panache45/32/64_2.png) [@panache45](https://boards.straightdope.com/u/panache45)\ **Post date:** [March 4, 2008, 8:10am UTC](https://boards.straightdope.com/t/bridge-on-the-river-kwai-why-does-the-water-go-down/439951/9 "2008-03-04T08:10:30Z")
There are many places that are completely dry river beds . . . until a flash flood comes sweeping through, almost instantaneously. ---
**Author:** ![muldoonthief](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/muldoonthief/32/3188_2.png) [@muldoonthief](https://boards.straightdope.com/u/muldoonthief)\ **Post date:** [March 4, 2008, 2:29pm UTC](https://boards.straightdope.com/t/bridge-on-the-river-kwai-why-does-the-water-go-down/439951/10 "2008-03-04T14:29:41Z")
The simple answer is that it was necessary to the plot, and at the time the movie was made there were no websites where people would post “Goofs” for movies, and no messageboards where fans could dissect every scene of the movie as well. No snark intended. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [March 4, 2008, 3:09pm UTC](https://boards.straightdope.com/t/bridge-on-the-river-kwai-why-does-the-water-go-down/439951/11 "2008-03-04T15:09:28Z")
Maybe there was a dam upriver. On Mohave lake, you can easily get your boat beached (or at least prop damaged) when they lower the flow out of Hoover dam. Don’t ask how I know… ---
**Author:** ![NetTrekker](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/nettrekker/32/2909_2.png) [@NetTrekker](https://boards.straightdope.com/u/NetTrekker)\ **Post date:** [July 24, 2022, 2:49pm UTC](https://boards.straightdope.com/t/bridge-on-the-river-kwai-why-does-the-water-go-down/439951/12 "2022-07-24T14:49:30Z")
And how did the POWs raise the heavy support logs for the bridge with no heavy equipment or animals? And who built the railroad that magically appeared once the bridge was finished? 😉 ---
**Author:** ![Hypno-Toad](https://avatars.discourse-cdn.com/v4/letter/h/b2d939/32.png) [@Hypno-Toad](https://boards.straightdope.com/u/Hypno-Toad)\ **Post date:** [July 24, 2022, 11:56pm UTC](https://boards.straightdope.com/t/bridge-on-the-river-kwai-why-does-the-water-go-down/439951/13 "2022-07-24T23:56:58Z")
There are many rivers that rise and fall significantly without warning. It’s possible that the high level of the river on the day the commandos arrived was the unusual situation and was due to heavy rains days earlier miles upstream. Once that surge of water was gone, the normal lower level was resumed. And the heavy logs could have been raised with a great wheel inside of which men walk to rotate it. Timbers like those used on the bridge would have been used to make cranes. ---
**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\ **Post date:** [July 25, 2022, 1:05am UTC](https://boards.straightdope.com/t/bridge-on-the-river-kwai-why-does-the-water-go-down/439951/14 "2022-07-25T01:05:22Z")
> [@NetTrekker](#): > > And how did the POWs raise the heavy support logs for the bridge with no heavy equipment or animals? Zombies! > [@NetTrekker](#): > > And who built the railroad that magically appeared once the bridge was finished? 😉 Also zombies! It’s a fourteen year old thread people! ---
**Author:** ![Hypno-Toad](https://avatars.discourse-cdn.com/v4/letter/h/b2d939/32.png) [@Hypno-Toad](https://boards.straightdope.com/u/Hypno-Toad)\ **Post date:** [July 25, 2022, 3:13am UTC](https://boards.straightdope.com/t/bridge-on-the-river-kwai-why-does-the-water-go-down/439951/15 "2022-07-25T03:13:40Z")
Well that explains why I’ve answered twice. ---
**Author:** ![Gatopescado](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gatopescado/32/5711_2.png) [@Gatopescado](https://boards.straightdope.com/u/Gatopescado)\ **Post date:** [July 25, 2022, 6:08am UTC](https://boards.straightdope.com/t/bridge-on-the-river-kwai-why-does-the-water-go-down/439951/16 "2022-07-25T06:08:05Z")
Madness! _Madness!!!_