Salt River Thames.

[john.moly]

How come if the sea has salt in it? when it reaches the river Thames. the Thames water is freash water, ie no salt in it.hope this question makes sence?

john.

[Quizmonster]

As with any river, when the tide flows into it, there will inevitably be an element of saltiness. I understand that the 'tidal' section of the Thames stretches as far upstream as Teddington Lock. So, you'll probably find salt traces at all times that far upriver. Clearly, beyond that, where there is only fresh water flowing downstream, the water is fresh.

[brachiopod]

Put simply - water can't flow uphill, so salt water (in a rising tide) can only penetrate upstraem as far as its own sea level. (ie where the river bed is above sea level, there will be no salt water ingress)

Where a river is 'tidal' (the Thames is tidal up to Teddington Lock), and water level rises and falls with the tides, doesn't necessarily mean that the water is s alt water. The tidal rise at the river mouth has a 'damming effect' on freshwater flowing down the river, thus the tidal rise upstream is a result of the 'piling up' of fresh water as much as anything.

Where there is a tidal salt water ingress up a river mouth, being denser than fresh water, the salt water tends to remain as a layer below the fresh water.

[gen2]

Of course the Thames has salt in it. What do you think happens to all that salt the councils spread on the roads every winter.

As far as the sea is concerned, the salt content remains almost constant as the amount of fresh water that evaporates from the sea surface is balanced by the amount of fresh water that returns to the sea in rivers after falling as rain.

[brachiopod]

Snap! QM !!

[brachiopod]

Slight correction, gen2.

Returning freshwater brings with it dissolved minerals etc (the solute load). When water evaporates off the sea, it is just water - the dissolved minerals are left behind, thus there is a continual increase in concentration of minerals / salts.

[Quizmonster]

Yes, an exact bit of synchrography, Brachiopod! Your response is much more scientifically precise than mine, but I think we both got the key point across. Cheers

[gen2]

Yes brachiopod, I did say remains almost constant. I didn't want to add unnecessary complications to the answer. Over geological time, salts are of course removed from the sea again as evaporite deposits.

[brachiopod]

Hmmm, don't see that as an "unnecessary complication" myself, more of a basic truth.

Stick solute-laden water in, and evaporate (to all intents-and-purposes) pure water out and you get net concentration of said solutes - hence the eternal child's question "why is the sea salty?".

Evaporites I would say were more of a complication in that its occurance (and its effect on net solute concentration of the oceans) is sporadic according to climatic / isostaic / tectonic reasons etc.

[Jay70]

Estuaries contain what is known as brackish water which is neither marine or freshwater but has a salinity somewhere in between. The salinity can fluctuate strongly between these two extremes.