More than 1,000 merganser ducks apparently became disoriented in dense fog and were swept over the American and Horseshoe Falls and into the gorge more than 150 feet below, the police said today.

An unknown number of them were killed, and hundreds that survived the plunge Monday were still marooned today on the ice below the cataracts, unable to get out of the gorge because of broken wings or feet.

Chief Bill Derbyshire of the Niagara Parks Commission Police said the ducks could not be rescued from the ice because ''it's extremely dangerous down there.''