I just bought a house this past November, and I noticed shortly after moving in that the backyard is almost always muddy.

There's a creek along the outside perimeter of the yard, which is sloped, so whenever it rains, the creek overflows and basically floods the backyard.

I did a little digging by the deck and hit concrete about six inches down, which I've never seen before. I know the previous homeowners did some work back there to prevent the basement from flooding and I'm assuming it's related to that.

Now that it's getting warm, I'd like to solve this mud issue but I'm really not sure where to start. It doesn't seem like something a drain would solve (in part because there already is one, and also because the muddiness is pretty spread out).

Any suggestions for how to tackle this?