Hole in a swale

Hi guys

in one of my client's property, there is a hole in a swale, emptying a large proportion of the water that the swale holds. It is a deep hole, and looks like a collapsing or like it has been dug by an animal.
The hole is at least 1 meter (3 feet) deep and 60 cm (2 feet) wide.

I am about to fill it with clay, compact it, and plant it with trees. But I am curious what strategy you would adopt?

Thanks