Koi pond advice. New to all this.

[Daemonshi]

The bottom of the waterfall gap is sitting at 18". With 12" of gravel, the water would need to rise another 6" before it will return to the pond. How far above the gravel should the fall be sitting? I can lower the edges all the way around to free up some liner.

 

[poconojoe]

If your liner is short, lower the walls. You don't need it to be 2 feet deep.

Why would you need to plunge cut? The top is open. What tool are you using to cut with?
Drill a hole large enough for your blade in one bottom corner. Stick your blade in and cut across horizontally.

 

[poconojoe]

The bottom of the waterfall gap is sitting at 18". With 12" of gravel, the water would need to rise another 6" before it will return to the pond. How far above the gravel should the fall be sitting? I can lower the edges all the way around to free up some liner.

The bottom of the waterfall should be at the height of the gravel.
You can have it higher than the gravel, but then your whole bog will have water above the gravel. If that's what you want, that's OK, but you don't NEED the water to be above the gravel.
Some plants may not like the water that high above their crown.
So, generally the water should be a slight bit just below the gravel.
Some people like to have some parts of the bog with water above the gravel for a particular effect and that's OK too.
I could have used a little bit more gravel in mine, so there are a couple spots where the water is above the gravel. I plan on getting a small amount of gravel to cure this eventually.

 

[addy1]

I could have used a little bit more gravel in mine, so there are a couple spots where the water is above the gravel. I plan on getting a small amount of gravel to cure this eventually.

I have low spots, high spots, lil rivers, some places the roots from years of growing have made a nice mat. I just ignore.

 

[Daemonshi]

Anyone have any idea what this red inflamed area is around my koi's gills?

It first showed up after a heavy rain, and was only on the left side. Now it has spread to the right side. She either hangs out where you see her in the pic (under the gazebo), or where the water splashes back in from the UV filter. She does come up quite a bit for air. There is another koi that seems to be normal. Can't possibly be low oxygen in the water, with the return from the return, rain every day, and 2 air stones in the pond. I did read that it may be an oxygen transfer problem, treatable with salt and medicated food.

I've already put 3 bags of salt in the pond, each one treats 400 gallons. It's about 1600 gallons, so I'm going today for 2 more. I'm sure the rain washed out quite a bit. Also going to check the store for some medicated food, but idk she hasn't been eating anything except for worms. So, I'm going to see if they have any worms while I'm there. At least she will eat something.

There are also some missing scales, that may mean something:

Those splotches have been there since we moved in back in Dec. She has been acting fine until the red around the gills showed up.