This is where you need specifically Rivers of London canon knowledge, but because of its and its watershed's size and volume/wildness, both of which would confer significant power, plus the presumed vast age of said genus locii and their comparative lack of interaction with large numbers of pre- or -post invasion humanity.

ETA: I mean, the Thames, in a London- or at least UK- centred fictional universe, is quite properly a Very Big Deal. Once I get to trying to superimpose those assumptions on my own native landscape, some things just sort of ... jump out, and that's one.

Now working out if lakes have genus locii or not. If they do, my villians have accidentally bitten off not just more than they can chew, but a piece of a steak which is still attached to an animal capable of eating both of them and bawling for dessert.