What plants can survive being flooded for several months a year and mowed in the summer?

Part of a friend's field is swampy in the Spring, but dries into lawn and is occasionally mowed in the summer.

What flowers could be planted there? I guess the ideal plant would be a swamp loving Spring Ephemeral or a decorative swamp grass that can survive occasional mowing.

Also, can anyone think of any swamp loving, native variety of clover with particularly pretty flowers?