I don't know what kind of liner you have or what kind of glue or plastic you'd want to add, but unless the materials are compatible I don't think that's a good idea.

I normally push rocks under the liner at the edges. Makes the stream a tiny bit less wide but deeper. Dirt works too. In most cases concrete would be overkill.

I'm not sure what your setup is--can you dig out underneath the liner, making the stream deeper? Or you don't have enough liner for that, either?

Given the time of year, check for leaves. It's amazing to me how much a couple leaves in the wrong place can mess up my stream, sending the water lapping over the banks...though the dirt subsides under my mounded up banks, too, and I've had to repair a couple of those.

If you absolutely have to add liner, it's doable, my stream's long enough I had to splice the liner several times.

Completely agree with water_bug. I sometimes use rocks and sometimes use small boards to push the liner higher.