Carole, it is good you are willing to do so much for your horses. Organic matter (thatch) can hold 7x its weight in water. Over a long period of time, you might find the organic material later has grown up and will soak up some of the excess water. This won't prevent the waterlogging though.
Some suggestions can be found in the link, which might help.
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http://www.horseslandwater.com/paddock/seasonal
* Sow waterlogging-tolerant pasture plants on wet areas.
* Consider including swales (i.e. broad, low contour banks) and tree plantings to slow down, absorb and disperse excess water movement down slopes.