Summarize:

Garden rocks and stones don't only add a great feature to your stream they help to weigh down your liner so don't be afraid to use heavy ones.
Once the liner is in place, secure and bed down the edges with soil, be sure to choose soil conducive to plant growth. If you have added steps to your stream you can build out the steps with garden slate allowing the edge of the slate to overhang creating a waterfall effect, the slate also hides the liner underneath. Use good size rocks. You can cover the excess liner and soil with small stones (we used bags of 'gold light') and decorate the area with driftwood, logs, slate, granite and beach stones.