how close to the creek? will the soil be constantly wet? if so, it is not smart to grow there. otherwise, you can dig a hole big enough to add in new soil. since clay is slow draining, you must make sure the roots can get enough air (pretty deep hole man)
is there water going through were you want to plant it? cause if it is its a bad idea but yea you can just dig out the clay and replace it with some soil.