[...] City Attorney Lynn Lorenson said the city has not been able to identify who owns one of the shacks and all of them have limited, if any, documentation of ownership. [...] because the structures are located in the river, the city has little ground on which it can force the owners to sell since the Wisconsin Department of Natural Resources has oversight of waterways and structures in them, said George Curtis, the Oshkosh attorney who is representing the Whittys and two other owners.