# Is there any river you can't see across on a clear day? **URL:** \ **Category:** Factual Questions\ **Created:** [September 10, 2010, 11:49pm UTC](https://boards.straightdope.com/t/is-there-any-river-you-cant-see-across-on-a-clear-day/553352 "2010-09-10T23:49:08Z")\ **Posts on this page:** 15\ **Page:** 1
**Author:** ![ultrafilter](https://avatars.discourse-cdn.com/v4/letter/u/3d9bf3/32.png) [@ultrafilter](https://boards.straightdope.com/u/ultrafilter)\ **Post date:** [September 10, 2010, 11:49pm UTC](https://boards.straightdope.com/t/is-there-any-river-you-cant-see-across-on-a-clear-day/553352/1 "2010-09-10T23:49:08Z")
More specifically, is there any part of any river outside of the delta that’s wide enough that someone with 20/20 vision can’t see the opposite shore on a clear day? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 11, 2010, 12:04am UTC](https://boards.straightdope.com/t/is-there-any-river-you-cant-see-across-on-a-clear-day/553352/2 "2010-09-11T00:04:20Z")
For a person of normal height on a flat surface, the horizon is about 4.7 km away. The Amazon River has a permanent width of 10 km at its widest, and can swell to as much as 48 km in the rainy season. So, yes, there is such a river. ---
**Author:** ![The\_Flying\_Dutchman](https://avatars.discourse-cdn.com/v4/letter/t/9fc348/32.png) [@The\_Flying\_Dutchman](https://boards.straightdope.com/u/The_Flying_Dutchman)\ **Post date:** [September 11, 2010, 12:06am UTC](https://boards.straightdope.com/t/is-there-any-river-you-cant-see-across-on-a-clear-day/553352/3 "2010-09-11T00:06:39Z")
That depends on what you want to see.The other shore when you are standing at the edge of one bank ?The Amazon river is your best bet. ---
**Author:** ![Shagnasty](https://avatars.discourse-cdn.com/v4/letter/s/9dc877/32.png) [@Shagnasty](https://boards.straightdope.com/u/Shagnasty)\ **Post date:** [September 11, 2010, 12:11am UTC](https://boards.straightdope.com/t/is-there-any-river-you-cant-see-across-on-a-clear-day/553352/4 "2010-09-11T00:11:56Z")
You might run into a definition problem with this one. The lower Mississippi River in Louisiana has some areas that aren’t well defined and there places that are so overgrown vegetation and swampland that the shore is hard to see across. That is part of the delta however so you may be asking about the widest river in the world. That may be Rýo de la Plata at Montevideo, Uruguay which is about 50 miles wide at that point and looks like a huge lake or Ocean but it is wider at some points by some definitions. ---
**Author:** ![John\_DiFool](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_difool/32/19543_2.png) [@John\_DiFool](https://boards.straightdope.com/u/John_DiFool)\ **Post date:** [September 11, 2010, 12:31am UTC](https://boards.straightdope.com/t/is-there-any-river-you-cant-see-across-on-a-clear-day/553352/5 "2010-09-11T00:31:05Z")
Atmospheric refraction might allow you to see trees etc. from a further distance than the strict reading of the trig will tell you. There’s the Dnieper River, with all its wide reservoirs, but of course those are artificial in origin. ---
**Author:** ![Left\_Hand\_of\_Dorkness](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/left_hand_of_dorkness/32/7156_2.png) [@Left\_Hand\_of\_Dorkness](https://boards.straightdope.com/u/Left_Hand_of_Dorkness)\ **Post date:** [September 11, 2010, 12:47am UTC](https://boards.straightdope.com/t/is-there-any-river-you-cant-see-across-on-a-clear-day/553352/6 "2010-09-11T00:47:23Z")
[This one](http://crystalinks.com/mexicolongestriver.html). [/obligatory smartass answer given after the real answers] ---
**Author:** ![Bytegeist](https://avatars.discourse-cdn.com/v4/letter/b/9dc877/32.png) [@Bytegeist](https://boards.straightdope.com/u/Bytegeist)\ **Post date:** [September 11, 2010, 12:56am UTC](https://boards.straightdope.com/t/is-there-any-river-you-cant-see-across-on-a-clear-day/553352/7 "2010-09-11T00:56:37Z")
The Saint Lawrence River is wide enough, for most of its length. ---
**Author:** ![Little\_Nemo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/little_nemo/32/3120_2.png) [@Little\_Nemo](https://boards.straightdope.com/u/Little_Nemo)\ **Post date:** [September 11, 2010, 2:58am UTC](https://boards.straightdope.com/t/is-there-any-river-you-cant-see-across-on-a-clear-day/553352/8 "2010-09-11T02:58:21Z")
Last month I was standing on the shore of Lake Ontario in Olcott, NY and I could clearly see the Toronto skyline. That’s a distance of over forty miles which is far greater than the width of any river. But others have pointed out that a more reasonable standard is being able to see the opposite shoreline and have given examples of rivers where that’s not possible. ---
**Author:** ![Cerowyn](https://avatars.discourse-cdn.com/v4/letter/c/82dd89/32.png) [@Cerowyn](https://boards.straightdope.com/u/Cerowyn)\ **Post date:** [September 11, 2010, 3:48am UTC](https://boards.straightdope.com/t/is-there-any-river-you-cant-see-across-on-a-clear-day/553352/9 "2010-09-11T03:48:12Z")
> [@Little\_Nemo](#): > > Last month I was standing on the shore of Lake Ontario in Olcott, NY and I could clearly see the Toronto skyline. Did you see me waving back? What’s the definition of “the delta?” I mean, clearly, the part of a river that coincides with the shore of the body of water into which it flows would be part of the delta, but how far up-river do you go before you decide it’s not the delta anymore? ---
**Author:** ![AK84](https://avatars.discourse-cdn.com/v4/letter/a/85e7bf/32.png) [@AK84](https://boards.straightdope.com/u/AK84)\ **Post date:** [September 11, 2010, 5:34am UTC](https://boards.straightdope.com/t/is-there-any-river-you-cant-see-across-on-a-clear-day/553352/10 "2010-09-11T05:34:06Z")
The Indus is about 16 km wide it its extreme lenght. ---
**Author:** ![jasonh300](https://avatars.discourse-cdn.com/v4/letter/j/46a35a/32.png) [@jasonh300](https://boards.straightdope.com/u/jasonh300)\ **Post date:** [September 11, 2010, 4:08pm UTC](https://boards.straightdope.com/t/is-there-any-river-you-cant-see-across-on-a-clear-day/553352/11 "2010-09-11T16:08:16Z")
> [@Little\_Nemo](#): > > Last month I was standing on the shore of Lake Ontario in Olcott, NY and I could clearly see the Toronto skyline. That’s a distance of over forty miles which is far greater than the width of any river. > > But others have pointed out that a more reasonable standard is being able to see the opposite shoreline and have given examples of rivers where that’s not possible. The horizon gets further away for every foot in elevation you are (or in this case, the elevation of what you’re looking at over the horizon). You could see the skyline of Toronto, but you couldn’t see the ground because it was below the horizon. The further away you go, the less skyline you would be able to see until the tops of the tallest buildings disappeared below the horizon. ---
**Author:** ![Little\_Nemo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/little_nemo/32/3120_2.png) [@Little\_Nemo](https://boards.straightdope.com/u/Little_Nemo)\ **Post date:** [September 11, 2010, 5:09pm UTC](https://boards.straightdope.com/t/is-there-any-river-you-cant-see-across-on-a-clear-day/553352/12 "2010-09-11T17:09:54Z")
> [@jasonh300](#): > > The horizon gets further away for every foot in elevation you are (or in this case, the elevation of what you’re looking at over the horizon). You could see the skyline of Toronto, but you couldn’t see the ground because it was below the horizon. The further away you go, the less skyline you would be able to see until the tops of the tallest buildings disappeared below the horizon. I understand how a horizon works. But my point was that I was seeing buildings that were on the opposite shore, which would qualify as seeing across the lake. ---
**Author:** ![Polycarp](https://avatars.discourse-cdn.com/v4/letter/p/82dd89/32.png) [@Polycarp](https://boards.straightdope.com/u/Polycarp)\ **Post date:** [September 11, 2010, 5:32pm UTC](https://boards.straightdope.com/t/is-there-any-river-you-cant-see-across-on-a-clear-day/553352/13 "2010-09-11T17:32:29Z")
A similar objection to **Little Nemo** ’s might be raised where mountains begin abruptly from plains or rolling country, as in Colorado’s Front Range or, IIRC, Mons Olympus on Mars – where contiguous countryside “winks out” at the horizon short of where the mountain(s) rise beyond it. This being a natural feature, the issue is even stronger than with manmade structures visible beyond the horizon. Is there any definitive statement on the meaning of “horizon” in such instances? ---
**Author:** ![NinetyWt](https://avatars.discourse-cdn.com/v4/letter/n/cc9497/32.png) [@NinetyWt](https://boards.straightdope.com/u/NinetyWt)\ **Post date:** [September 11, 2010, 11:37pm UTC](https://boards.straightdope.com/t/is-there-any-river-you-cant-see-across-on-a-clear-day/553352/14 "2010-09-11T23:37:55Z")
You also need to define the stage of the river in question. There are obviously rivers which, at flood stage, are too wide to see across. ---
**Author:** ![NinetyWt](https://avatars.discourse-cdn.com/v4/letter/n/cc9497/32.png) [@NinetyWt](https://boards.straightdope.com/u/NinetyWt)\ **Post date:** [September 11, 2010, 11:43pm UTC](https://boards.straightdope.com/t/is-there-any-river-you-cant-see-across-on-a-clear-day/553352/15 "2010-09-11T23:43:37Z")
> [@Cerowyn](#): > > What’s the definition of “the delta?” I mean, clearly, the part of a river that coincides with the shore of the body of water into which it flows would be part of the delta, but how far up-river do you go before you decide it’s not the delta anymore? > [@](#): > > Delta definition: Coastal accumulations, both subaqueous and subaerial, of river-derived sediments adjacent to, or in close proximity to, the source stream, including the deposits that have been secondarily molded by various marine agents, such as waves, currents, or tides. [L. D. Wright, 1978] From [here](http://www.americaswetlandresources.com/background_facts/detailedstory/RiverDelta.html). The up-river limit would be that place where this type of sedimentation stops.