Owners of the nation's first riverboat-horseracing facility announced its closure Tuesday, minutes after the inaugural race. 'I guess we planned it pretty poorly,' said Ronald Frisch, president and CEO of Gambling Concepts Unlimited. 'We figured that once we opened the dining-room doors, the horses would know to race through the grand buffet room to the other side of the boat.' Twenty-five people were trampled to death in the chaos that resulted, and eight horses drowned when they fell from the riverboat's lower deck. Gambling Concepts Unlimited officials said they still plan to hold next month's airborne rodeo as scheduled. 