how big is this lake? to support an entire species you would need a stable food source in sufficient quantity for a creature of this size, you couln't have just one or even just one family, as inbreeding and the ocasional bad season or calamity would have wiped them out by now. You would need a rather large number to sustain the species and if there was a large number (say 10+ at least) there would probably have been more sightings. Is there any history of such creatures in local native american legends? that would be a good indicator as they could be a species that has declined recently and so that would explain the small number of individuals. also are there any other large aquatic andimals in the area if you can get a whale in the thames, what other lost sea creatures could end up there?