h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I have started fishing for course { ie. freshwater } fish in a tidal stretch of river . The rise and fall of tide is around 4 meters . Now from what i have gleaned from various places is that saltwater , being denser than fresh , slides under the freshwater in a kind of tunnel or wedge forceing the freshwater to the surface . What i would like to find out is how far does the salt water penatrate up the estuary . Does the saltwater stay hard on the bottom pushing way up the river in a long thin finger , or is it in a steep wedge pushing all the freshwater in front of it ?
Anybody got any ideas or have any ideas where i might find this kind of information ?

best fishes......tod

I believe that a long thin stream of salt water will go up the river.

I know that in the Bosporus, which is the strait between the strongly salty Mediterranean and the 1/3 salty Black Sea, there are two separate currents, one on the surface in one direction and one on the bottom in the other direction. This suggests that the two different types of water can keep separate for long distances.

I've also heard that in the estuary of the Boyne river in Ireland, they built a Magnesium plant which extracts magnesium from seawater, and that they were able to build it much further upstream than people thought possible because they did a detailed hydrological study of the river and found that the sea water went further up than people thought.

Ok , I have been digging around on the web and I have found that the river Thames is esentualy freshwater from London bridge upwards {to Tedington lock } The river I am researching is the Medway , which is very simular but on a smaller scale . As to how far the saltwater penetrates into the freshwater . Each estuary is unique and it can be an allmost horizontal stratification to an almost vertical wall . Plus it is dynamic with variables such as the amount of freshwater flowing out of the river and how big the tide is etc. It is a very interesting enviroment .

Best fishes........tod

I don’t know the answer to your question but one of the most fascinating books I ever read, concerned the exploits of the WW2 commando frogmen who manned the two seat human torpedoes or chariots. Many of their practice dives were carried out in Scottish lochs and one of the main hazards they had to contend with was when salt and fresh water mixed. The book suggested that it was an haphazard, uneven process resulting in varying pockets of salt/fresh water and it was one of the divers greatest fears.
The craft were trimmed to one or the other but as they moved through the pocket the variability in density of the water would cause the craft to dive or ascend rapidly resulting in all manner of injuries including burst eardrums and oxygen poisoning.

Interesting thread guys

Keep it going

Well i have done a bit of research and have discovered that the Thames is essentualy freshwater from tower bridge up . The river i am interested in is the Medway which is close by and of a very simular nature .
I have had a couple of goes at catching fish this week , nothing so far . However yesterday the river flowed strongly on the flood tide for a few hours and then completely stopped for 20 minutes before once again continuing to flood . The only explenation i can come up with for this is that there is somekind of backwash when the tide comes up against the first lock {Allington} .

Best fishes.....tod

Note to self ; read what you have posted previousely so as not to repeat yourself.


best fishes...tod