Follow up. The cutgrass became submerged and died off over the winter. I cut and raked a ton of it out of the pond and burned it.

I installed a drainage pipe this spring, so the water is now at a constant level and not rising any further.

The cutgrass is back, thick as before in some spots, but thinner in others.

I did a test with a 3% solution of Airmax Shoreline Defense (53% glyphosate) as recommended on the product page. It seemed to be effective.

After reading the label more closely, it seems 3/4% to 1% solution is the recommended amount for cutgrass and cattails. I will give that a go.

[Linked Image from i.imgur.com]