Several times I've seen this Thames Water boat go up and down the river and wondered what it is. Now I know, from a program called "Thames: wildlife superhighway" which was broadcast here during the week.
In this week's episode they went on board this interesting boat to find out what it does, why it does it, and how it does it.
It turns out this is a special boat that pumps oxygen into the river at times when levels dissolved in the water gets low. For example if it rains a lot (like it did this summer) it can exceed the ability of the sewage network to get rid of it. This is a legacy of Victorian London, in that the same networks of pipes is used to carry away both rain water and sewage.
In times of really heavy rain it becomes necessary to let raw sewage enter the river (sorry about this theme, but at least you now know why there is a very good reason NOT to drink it). As it decays it sucks oxygen out, which can kill thousands of fish.
Hence the boat: at times of such stress to the wildlife the boat is despatched out to pump life back into the river.
It may look expensive to have such a dedicated vessel, but the alternative would be creating a whole new network of pipes (we're talking billions) or have the river life be killed off at regular intervals.
I'm glad its there - but now I know what it does I'll be wondering each time I see it why exactly its out there!