River crossing

[copper_head]

Id read or been told that if a river is too dangerous to cross it is best to wait until dawn, as the flow will be at its lowest. I also noticed that this happened on Operation: Grand Canyon, they waited until morning and were able to free the trapped boat.

So does anyone know why river levels drop at night?

 

[SJStuart]

If the river is tidal, then its level will be linked with the state of the tide (naturally). Estuaries are a perfect example of this (such as the River Alde, the River Orwell, and any other rivers which feed directly into the sea rather than tributaries of larger rivers).

Also, if a river has been damned or has sluice gates (even lochs) then its level will be influenced by the operation of those man-made obstacles.

Damned rivers will have their level lowered over night while water is pumped back to the top of the dam. This is most common for hydroelectric dams, where the water is pumped back up at night because the electrical demand is at its lowest (thus the surplus can be used for the task of ensuring the water level at the top of the dam is at its highest before peak demand the following day)

EDIT: The flow rate of rivers is also influenced by the moon in the same way as the tides.

 

[John Fenna]

In mountainous regions where the summits are snow covered the waters freeze up at night giving a reduced flow in the mornings. As daylight and the sun melts the snow then the flow increases again.

 

[copper_head]

Thanks sjs yeah I considered tidal and man made influences. But I got the impression this phenomena was more or less universal. So perhaps it is the moon's influence.

 

[SJStuart]

In mountainous regions where the summits are snow covered the waters freeze up at night giving a reduced flow in the mornings. As daylight and the sun melts the snow then the flow increases again.

Another very good point I hadn't remembered, cheers John

 

[bearbait]

Glacial and/or snow-fed rivers will increase their flow during the day as the sun melts the snow & ice further upstream, and will slow down overnight as the temperature drops again and the rate of melt decreases or even stops.

 

[SJStuart]

Also worth noting that water expands when it gets warmer, and contracts when it gets colder... so melt or no melt, just increasing the temperature of the water will increase its flow rate (larger volume in limited channel = faster rate of flow) and decreasing the temperature will cause the inverse.

 

[Pterodaktyl]

Thanks sjs yeah I considered tidal and man made influences. But I got the impression this phenomena was more or less universal. So perhaps it is the moon's influence.

The moon could have an effect, but since the moon doesn't rise and set with the sun (ie. you can have nights when the moon is below the horizon and days when the moon is visible) there wouldn't be any link between the moon's influence on the water level and the time of day. I'm going with the water contracting/expanding due to temperature changes, and it's usually coldest just before dawn.

 

[copper_head]

In mountainous regions where the summits are snow covered the waters freeze up at night giving a reduced flow in the mornings. As daylight and the sun melts the snow then the flow increases again.

Glacial and/or snow-fed rivers will increase their flow during the day as the sun melts the snow & ice further upstream, and will slow down overnight as the temperature drops again and the rate of melt decreases or even stops.

Thanks gents that certainly makes sense.

Also worth noting that water expands when it gets warmer, and contracts when it gets colder... so melt or no melt, just increasing the temperature of the water will increase its flow rate (larger volume in limited channel = faster rate of flow) and decreasing the temperature will cause the inverse.

Hmmm, hadn't considered that. Would the diurnal variation in temp really cause that much expansion I wonder?

Some good info here though, perhaps its a case of a many contributing factors being generalised into one 'rule'.

 

[SJStuart]

Hmmm, hadn't considered that. Would the diurnal variation in temp really cause that much expansion I wonder?

Some good info here though, perhaps its a case of a many contributing factors being generalised into one 'rule'.

Think about it: longer the river (and the more water it contains) the greater the overall degree of expansion. Indeed, the further down-stream from the source you are, the greater the flow rate will be as there's a greater volume expanding above accelerating the rate of flow.

Even a half Celsius rise in temperature would equate to a highly significant increase in flow rate.

That would quite happily explain why the rate of flow peaks with the temperature, and is at its slowest when the temperature is at its lowest (right around dawn)

 

[SJStuart]

The moon could have an effect, but since the moon doesn't rise and set with the sun (ie. you can have nights when the moon is below the horizon and days when the moon is visible) there wouldn't be any link between the moon's influence on the water level and the time of day. I'm going with the water contracting/expanding due to temperature changes, and it's usually coldest just before dawn.

It's probably fair to say the moon would have more of an affect on the static level of the water rather than its rate of flow... though it wouldn't be the most important factor given all the other natural and man-made variables at play (such as ice melt, dams etc)

 

[copper_head]

Think about it: longer the river (and the more water it contains) the greater the overall degree of expansion. Indeed, the further down-stream from the source you are, the greater the flow rate will be as there's a greater volume expanding above accelerating the rate of flow.

Even a half Celsius rise in temperature would equate to a highly significant increase in flow rate.

That would quite happily explain why the rate of flow peaks with the temperature, and is at its slowest when the temperature is at its lowest (right around dawn)

I think you have my answer, found an article that shows just this.

http://ir.library.tohoku.ac.jp/re/bitstream/10097/44931/1/AA0045945070249.pdf

Thanks all

 

[SJStuart]

I think you have my answer, found an article that shows just this.

http://ir.library.tohoku.ac.jp/re/bitstream/10097/44931/1/AA0045945070249.pdf

Thanks all

Trust the Japanese to have a thesis on this very phenomenon

This is why I love physics

Just read the thesis... and it's actually quite brilliant in terms of clearly illustrating temperature's affect on the flow rate of water over distance.
Genius of them to include a section bisected by a hydro facility (which artificially increases the temperature) to demonstrate that it isn't merely a correlation, but rather a causation of temperature on flow rate.

I think it's safe to say Isao Ishikawa just won this thread (even though the paper was produced in the late 60's)

 

[ozzy1977]

Why does the Mediterranean have no tide?

 

[Badger74]

When I've been on holiday, it had one then.

Sent from my KFOT using Tapatalk 2

 

[SJStuart]

Why does the Mediterranean have no tide?

It does have tides, but since the affect of a tide (amplitude) is relative to the volume of a singular body of water, the observed affect is substantially lower than with comparatively massive volumes such as the Pacific and Atlantic. Further, the Mediterranean is subject to underwater currents from the Strait of Gibraltar, which further impact on its "tidal" condition.

I'm not sure where the myth originates that the Mediterranean has no tide... but it is just that: a myth. It might not have much of a noticeable tide, but even a few centimeters of difference is still a tide

I'll tell you somewhere that doesn't have a tide: Southport Beach. The tidal condition there seems to be "perpetually out"

EDIT: Decided to add a link to some better information on the subject, since they explain it better than I can.

 

[Uilleachan]

Other than the stuff about dams etc, I don't find dawn heralding anything special in terms of water level in rivers, good for fishing though

On the Tummel in Perthshire, major tributary of the mighty Tay, dawn is generally when the level is at it's highest as the dam at Pitlochry usually lets some water out first thing to move salmon and grilse up to the ladder for the tourists to oggle at from the observation station later in the day.

I guess snow melt in the arctic/subarctic north may slow during the night, but as to influence on levels, that would depend how far down stream of the melt one was. More than 30-35miles and I could see that not really making much difference as you'd be ahead of the melt delay, plus there'd be more local influences to consider. In the arctic/subarctic north one would want to be early or quite late in the season to take advantage of lower river levels. Through the summer months the water tends to be at it highest.

In scotland, where I've done most of my fording wading and fishing, I keep quite a close watch on river levels because I'm usually fishing, and perhaps looking to cross somewhere, that and the fish tend to run just as the water rises and again when it drops.

Some rivers, on skye for example, can go from raging torrent to gentle stream within an hour of the rain stopping, due to the limited reach from source to the sea, a max of around 6 miles because one is never anywhere on the island more than that from the coast, plus skye has a really steep run off too. Unless they're having a massive thaw on the skye hills, snow melt has only a small influence as the hills hold limited amounts. Rain, and the seemingly endless deluge has by far the biggest effect on levels.

Rivers local to me are similar but tend to drain larger areas and while the ground is steep, rising and lowering is a more gradual slower affair, this is the western watershed which is very short compared with that on the east, the western water shed on local rivers is 20 miles maximum with 10 miles being more typical. On the east, the Spey and Tay, in particular, the sources are more or less on the west-coast or not far then crossing the width of the whole country, meaning the rivers can rise and fall due to influences many many miles away. Anywhere half way down those rivers one is looking for a bridge or a boat, to get across.

The biggest problems I've had with fording running water has been on tiny streams that turn into impassable obstacles in heavy rain, I've gone up the hill fishing forded up to my ankles and found the same stream impassable on my return. I reckon of all the things we get up to in the great outdoors, river/stream crossing has to be about the most hazardous of ventures. It doesn't take much to wash you off your feet, and once in it can be quite a task to get back out. Most don't make it.

 

[santaman2000]

The rivers here in the Gulf South aren't really influenced by whether it night or day. The water temp remains mostly the same (within 5*-10*) year round, and the change in temp from night to day is probably less than a full degree, so expansion/contraction would be negligible. Snow melt? What's that? There is a tidal influence, but only for the first mile or so upriver from the mouth on most rivers, a bit more on the larger ones such as tha Mississippi or the Alabama Rivers.

Even when I was back out west there was no discernable difference in water level from night to day, as such, on free running rivers. The heavily dammed ones obviously were influenced by the floodgate schedule releasing water downstream. Equally obviosly, they had no tidal inflence whatsoever, being hundreds (if not a thousand) river miles upstream and thousands of feet above sea level.

 

[Panama Jungle]

In this neck of the jungle river levels are influenced by rain. I have seen a river go, in a matter of minutes, from a crystal-clear gently flowing stream a few inches deep to a raging chocolate-coloured torrent a few metres deep that would kill anything in its path, because it had rained heavily upstream. Sometimes I have been cut off because the river I crossed so easily in the morning was completely un-crossable on the way back in the afternoon. Time to sling a hammock and relax until it goes down...

 

[Panama Jungle]

In this neck of the jungle river levels are influenced by rain. I have seen a river go, in a matter of minutes, from a crystal-clear gently flowing stream a few inches deep to a raging chocolate-coloured torrent a few metres deep that would kill anything in its path, because it had rained heavily upstream. Sometimes I have been cut off because the river I crossed so easily in the morning was completely un-crossable on the way back in the afternoon. It often rains more in the afternoon than in the morning in the tropics. Time to sling a hammock and relax until it goes down...