Puzzled by frogs

Rietvlei 7 11 2008

About two hundred river frogs appeared in Santie's backyard pool, bordering on the vlei, after a 5 mm rain shower the night before.

They were identified as

Clicking stream frogs

Banded stream frogs

Common river frogs

We managed to catch most of them. The frogs were released into the vlei.

The question is

"How did 200 fully grown frogs end up in a fully enclosed, well maintained, backyard pool when there is no other water source within he enclosure?"