The River Thames in London flows both ways!
This is not technically correct it always flows from source to mouth, but because it is actually tidal all the way from the sea to Teddington where the first lock can be found it does flow in as well as out!
The tide is effectively the rise and fall of the sea (river) caused by gravitational forces as the Earth spins – more here: Tide explained on Wikipedia
As the tide out at the coast rises and falls it affects the rivers and in London actually causes a 5-7metre twice daily rise and fall. As you can imagine this is a large amount of water to be removed and added which results in some very strong currents downstream with the flow and then upstream when the tide is rising.
This has a huge affect on all that surrounds the river, both visually and on navigation. The amount of water affects what the fixed walls, buildings and piers look like and the change of direction of flow and strong currents controls navigation and boat movements.
The tide affects the speed the London Party Boats and Thames Passenger Boats can travel on the river. When going with the flow of the tide they can travel faster and when against the tide often around half the speed. This is obviously not a problem if you are in a speed boat, or Jet boat, but slower vessels travelling larger distances can have at least double the journey time, and of course increased fuel costs, all as a result of the tide.
Above Teddington lock all is peaceful with a steady predictable and controlled flow where the tide has no affect, it is only below the lock where it can be more interesting. Where the wind is against the direction of the tide the water is whipped up causing it to be a little bumpier – fine on the larger Thames boats, but no fun in a small sailing dinghy! Swimming in the river is therefore obviously not advised particularly when the tide is flowing at it’s strongest.
The tide can also be useful – when one of the last remaining Concorde’s had to be relocated from Hammersmith to Scotland a barge was brought up the river through London using the low tides to navigate under the bridges. This feet of planning was only possible over 2 separate tides for both the inbound and outbound journey!
Due to the tide the River Thames in London is a diverse and ever changing exciting location – untamed by human activity right in the heart of the City.