First, it needs to be determined whether you actually have a leak or if the water loss is just due to evaporation. See our evaporation article to calculate what is normal for your specific water feature. It’s fairly typical for a pond to lose 1 to 1.5 inches of water per week just from evaporation. If your pond is losing more than that then follow the steps below to pinpoint where the leak is coming from.
Step 1: Take a close look around the stream/waterfall and pond for any obvious wet spots. The most common cause of leaks is settling at the liner edges which allows water to spill over the side. If you find a leak like this, usually the liner can simply be pulled up higher and secured by backfilling with more dirt, rocks, or gravel. If there are no obvious wet spots move on to step 2.
Step 2: Turn off the pump(s), fill up the pond to the maximum water level, and wait at least 24 hrs to see if the pond itself is holding water. (If you have an aerator it would be a good idea to run it so the water stays oxygenated for your fish.)
Step 3: If the pond itself holds water, then the leak must be coming from either the plumbing or somewhere in the stream/waterfall. It's time to start a process of elimination so you're not looking for a needle in a haystack.