I wouldn't bother trying to fill it with your native soil. Judging from the rest of your yard, you are not lacking for planting spaces with native soil. People spend hundreds or thousands of dollars to create a specialized bog garden; you already have one! Take advantage of it. Plant it full of Louisiana irises, which love growing in soggy soil. It will be gorgeous! There are also many other types of interesting bog plants if you want a mixture. Browse places online that sell pond plants to see what is available. I am so jealous that you have this. I can't believe you're trying to get rid of it.
