# What determines the speed of flowing water at any given point? **URL:** \ **Category:** Factual Questions\ **Created:** [May 28, 2024, 1:07am UTC](https://boards.straightdope.com/t/what-determines-the-speed-of-flowing-water-at-any-given-point/1002165 "2024-05-28T01:07:34Z")\ **Posts on this page:** 11\ **Page:** 1
**Author:** ![ekedolphin](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ekedolphin/32/10003_2.png) [@ekedolphin](https://boards.straightdope.com/u/ekedolphin)\ **Post date:** [May 28, 2024, 1:07am UTC](https://boards.straightdope.com/t/what-determines-the-speed-of-flowing-water-at-any-given-point/1002165/1 "2024-05-28T01:07:34Z")
When a mass of water is flowing down a river, or in a creek, stream, or lake, for example, what determines the speed at which it flows? ---
**Author:** ![dolphinboy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dolphinboy/32/330_2.png) [@dolphinboy](https://boards.straightdope.com/u/dolphinboy)\ **Post date:** [May 28, 2024, 1:38am UTC](https://boards.straightdope.com/t/what-determines-the-speed-of-flowing-water-at-any-given-point/1002165/2 "2024-05-28T01:38:04Z")
Nevermind. ---
**Author:** ![Tabco](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tabco/32/7644_2.png) [@Tabco](https://boards.straightdope.com/u/Tabco)\ **Post date:** [May 28, 2024, 1:39am UTC](https://boards.straightdope.com/t/what-determines-the-speed-of-flowing-water-at-any-given-point/1002165/3 "2024-05-28T01:39:54Z")
For open channel flow (i.e. not a pipe or something like that), Manning’s equation is often used to model flow velocity. The factors affecting velocity in that equation are the “hydraulic radius” which covers channel size and form factor, Manning’s _n_ which describes how rough the surface of the channel is, and the slope of the channel itself (the steeper the channel, the faster the water moves). You can read about it at the link below. It’s a little bit math-intensive though. > **[Manning formula](https://en.wikipedia.org/wiki/Manning_formula)** > > The Manning formula or Manning's equation is an empirical formula estimating the average velocity of a liquid in an open channel flow (flowing in a conduit that does not completely enclose the liquid). However, this equation is also used for calculation of flow variables in case of flow in partially full conduits, as they also possess a free surface like that of open channel flow. All flow in so-called open channels is driven by gravity. > It was first presented by the French engineer Philippe Ga... I would wager that using just Manning’s equation is a bit simplistic for modeling flow in something like a river or lake. ---
**Author:** ![Civil\_Guy](https://avatars.discourse-cdn.com/v4/letter/c/d78d45/32.png) [@Civil\_Guy](https://boards.straightdope.com/u/Civil_Guy)\ **Post date:** [May 28, 2024, 2:54am UTC](https://boards.straightdope.com/t/what-determines-the-speed-of-flowing-water-at-any-given-point/1002165/4 "2024-05-28T02:54:41Z")
A bit simplistic, yes. The Manning formula only gives the average velocity, but I’d say that anyone watching the river run can see that the surface water in the middle is moving faster than the water at the banks. As I recall from college, the water somewhat below the surface moves slightly faster than the water at the surface. So the reality is complicated (a general thing with reality). ---
**Author:** ![MrDibble](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mrdibble/32/114_2.png) [@MrDibble](https://boards.straightdope.com/u/MrDibble)\ **Post date:** [May 28, 2024, 12:26pm UTC](https://boards.straightdope.com/t/what-determines-the-speed-of-flowing-water-at-any-given-point/1002165/5 "2024-05-28T12:26:31Z")
Which use of “determines” are you intending: What causes the speed? or How can you calculate the speed? > [@Civil\_Guy](#): > > As I recall from college, the water somewhat below the surface moves slightly faster than the water at the surface. It can vary quite a bit. That does happen, but generally not appreciably: > **[e3sconf\_riverflow2018\_06015.pdf](https://www.e3s-conferences.org/articles/e3sconf/pdf/2018/15/e3sconf_riverflow2018_06015.pdf)** > > 904.29 KB ---
**Author:** ![ekedolphin](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ekedolphin/32/10003_2.png) [@ekedolphin](https://boards.straightdope.com/u/ekedolphin)\ **Post date:** [May 30, 2024, 2:11am UTC](https://boards.straightdope.com/t/what-determines-the-speed-of-flowing-water-at-any-given-point/1002165/6 "2024-05-30T02:11:23Z")
Yes, what causes the speed, besides obviously gravity? ---
**Author:** ![MikeF](https://avatars.discourse-cdn.com/v4/letter/m/45deac/32.png) [@MikeF](https://boards.straightdope.com/u/MikeF)\ **Post date:** [June 2, 2024, 12:40pm UTC](https://boards.straightdope.com/t/what-determines-the-speed-of-flowing-water-at-any-given-point/1002165/7 "2024-06-02T12:40:37Z")
My uneducated guess would be gravity is the only cause of the speed. No gravity, no velocity. Other factors may impact the speed but wouldn’t be the “cause”. I guess water being released from a pressurized container in a zero-G environment would have velocity, based on the pressure difference and the size of opening in the container but I don’t think that’s what you asking. I’ve rarely seen measurements of waterflow in a river measured in MPH. Usually, it cubic feet per second. That said, there is a narrow man-made canal in NJ with high flow rates during tidal changes. It can get very rough. (look up Point Pleasant Canal on YouTube) I used to work as a marine police officer stationed on the canal and, while I don’t recall ever seeing official numbers, peak water speeds were over 6 knots, IIRC. That would be an important number to know if your vessel can make only 8 knots I don’t know why it isn’t measured and publicized in real time. Here’s a USGS link of measuring stream flow. [https://www.usgs.gov/special-topics/water-science-school/science/how-streamflow-measured](https://www.usgs.gov/special-topics/water-science-school/science/how-streamflow-measured). ---
**Author:** ![aceplace57](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/aceplace57/32/3500_2.png) [@aceplace57](https://boards.straightdope.com/u/aceplace57)\ **Post date:** [June 2, 2024, 1:31pm UTC](https://boards.straightdope.com/t/what-determines-the-speed-of-flowing-water-at-any-given-point/1002165/8 "2024-06-02T13:31:22Z")
I’ve wondered why some streams are docile and others turn into white water rapids. The ones that get swimmers into trouble look safe on top and have strong currents underneath. There has to be a complex combination of factors. Slope seems obvious. Water runs faster flowing downhill. That’s just one of many factors. ---
**Author:** ![Crane](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crane/32/3495_2.png) [@Crane](https://boards.straightdope.com/u/Crane)\ **Post date:** [June 2, 2024, 1:50pm UTC](https://boards.straightdope.com/t/what-determines-the-speed-of-flowing-water-at-any-given-point/1002165/9 "2024-06-02T13:50:28Z")
Well ISTM that the end points are a lake and a waterfall. The flow rate in the lake is minimal and the flow rate in the waterfall is maximal. So the flow rate in a stream is determined by the number, and shape, of objects impeding progress and how closely it’s angle of descent resembles a waterfall. ---
**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:** [June 3, 2024, 12:48pm UTC](https://boards.straightdope.com/t/what-determines-the-speed-of-flowing-water-at-any-given-point/1002165/10 "2024-06-03T12:48:07Z")
One simple and fundamental principle is that if you take any bounded volume of the river, if the river is in a steady state (which it usually will be), the amount of water flowing into that volume per time will be equal to the amount flowing out of that volume per time. So if you have a wide, deep section of river, and then a narrow, shallow section, the water must be flowing faster, on average, through the narrow, shallow section, to get out all of the water that’s coming in through the wide, deep section. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [June 4, 2024, 11:51pm UTC](https://boards.straightdope.com/t/what-determines-the-speed-of-flowing-water-at-any-given-point/1002165/11 "2024-06-04T23:51:31Z")
And it’s when that steady state assumption is violated that we get floods. Every river section has some maximal volume it _can_ flow. As the total water flow increases, eventually it hits the limit of the most limiting section. At which point upstream water is coming in faster than downstream water can vacate. So relatively high energy water begins backing up and real soon the water level(s) in the upstream section(s) rise and water departs the channel heading into town, farmland, or forest.