Anytime!

For weighing down the pond liner having an edge or lip about 1/2 meter over the stream with rocks and soil piled on top would suffice.

You could also do an Anchor Trench, you do this by extending the liner out about 2 meters out beyond the lip and dig a trench and put the excess pond liner in taut and fill the trench with heavy rocks and fill over that with soil. This way is extremely secure and you could even put rocks along lip of the stream as well to have it even more secure and prevent kids from trying to get into the stream.

When it comes time for monsoon and rain season, I'd empty a good bit out beforehand and then have a sump pump handy and attach a hose to it to run it out far away from the stream so you prevent the liner from lifting.