A "--" in the district column means, that the town is a district-free town, i.e. it is by itself a district.
... Census Bureau, the town has a total area of 4.4 km² (1.7 mi²). 3.3 km² (1.3 mi²) of it is land and 1.1 km² (0.4 mi²) of it is water. The ...