Sunday Bathtime Stupid Question

I do love a nice long soak in a hot bath of a weekend.

I don't have the time during the week; it's strictly showers only, so when the weekend rolls around I find myself called by the deep, primæval urge to soak; to luxuriate in the sensual yumminess of my bathtub and to let my mind wander whither it will.
A big mug of coffee and maybe a few squares of Green and Black's chocolate and I drift off into realms of weirdness unplumbed outside of that steamy haven.

Last week I lay in the bath thinking about Jesus - and you know what that led to.
Next up, it was quantum mechanics … Hey, I happen to like quantum mechanics! And I am fortunate enough to possess the arithmetical chops required to ponder the subject in the bath. It is not boring!

Today it was - somewhat more prosaically - "How the fuck did all the fish get to the Lake District?"
Hokay, so it's rather more mundane than Jesus or quantum mechanics, but my mind has many and varied dusty corners and that's what came creeping out this morning.
So: freshwater fish …
We have river fish and we have lake and pond fish. And a lot of these lakes and ponds are ultimately connected into river systems that reach the sea. So I can see how fish got into those.
But …

The Lake District.
Ten thousand years ago, at the end of the last ice age, the glaciers retreated to leave a series of valleys with pretty melt-water lakes in the bottom. All very picturesque and great fun for old Wordsworth and his chums, but not in any way connected to a river system (okay, Phil; I know that technically only one of them is a "lake" and the others are "waters", but semantics aside, the point I am trying to get across here is that they are a just bunch of overambitious ponds with no aqueous link to any possible piscine parental source).

So somebody tell me please, how the fuck did they get to be so full of fish?

Don't try to tell me Og and Ug decided to catch loads of river fish in their stone nets, put them in their stone buckets and haul the buggers up to the inhospitable heights of the Lake District and tip 'em in there.

And don't tell me God did it. There's not a HA! in the whole wide world big enough to pour scorn on THAT one!

Just please, for my Sunday sanity's sake, give me a plausible explanation.