Semily District briefly borders Poland in the north. The terrain is very diverse, mountainous in the north, foothills in the middle and slightly undulating in the south. The territory extends into five geomorphological mesoregions: Giant Mountains Foothills (most of the territory), Giant Mountains (north), Ještěd–Kozákov Ridge (a strip from west to south) and Jičín Uplands (southwest). The highest point of the district and of the entire Liberec Region is the mountain Kotel in Vítkovice with an elevation of 1,435 m (4,708 ft), the 10th highest mountain in the country. The lowest point is the river bed of the Jizera in Všeň at 236 m (774 ft).
From the total district area of 662.3 km2 (255.7 sq mi), agricultural land occupies 368.8 km2 (142.4 sq mi), forests occupy 229.0 km2 (88.4 sq mi), and water area occupies 6.6 km2 (2.5 sq mi). Forests cover 23.2% of the district's area.[1]
The most important river is the Jizera, which crosses the territory from north to southwest. Its longest tributaries in the district are the Oleška and Jizerka. The Cidlina originates here, but soon after leaves the district. The area is poor in bodies of water.
Most of the Giant Mountains area of the district is protected as the Krkonoše National Park and belongs to the most valuable area of the country thanks to a significant amount of rare flora and fauna. In the west, two separate parts of the Bohemian Paradise Protected Landscape Area lie within the district. A small part of the Jizerské hory Protected Landscape Area also extends into the territory, although the Jizera Mountains range itself does not extend into the district.
The D10 motorway from Prague, which further continues as the R/35 expressway to Liberec, runs along the western border of the district. The most important roads in the district are the I/10 road (part of the European route E65) from Turnov to the Czech-Polish border, the I/35 road (part of European route E442) to Hradec Králové, and the I/14 road from Liberec to Trutnov.
