Údolí Jizery u Semil a Bítouchova, Nature reserve in Semily, Czech Republic.

Údolí Jizery u Semil a Bítouchova is a canyon about three kilometres long cut by the Jizera River into metamorphic rocks, mainly metadiabase and green schist, roughly two kilometres north of Semily. The reserve was declared in 1951 and enlarged and re-declared by Liberec Region regulation 2/2013 to about 1,245,072 m². Its walls rise to 130 metres, and the basalt Krkavčí skála, over 100 metres high, carries an iron lookout.

The reserve was first declared in 1951 on 39.28 hectares. In 2013 Liberec Region regulation 2/2013 re-declared and enlarged it as Údolí Jizery, with an area of 1,245,072 m² (1,230,378 m² in the AOPK ČR register), though the old name is still used. The oldest human works in the valley are four railway tunnels opened in 1859, built with over 50 tonnes of gunpowder.

The Riegrova and Kamenického paths on the right bank, and the yellow-marked route on the left, are named after figures of Czech tourism; the Böhmova vyhlídka honours Antonín Böhm, first chairman of the Semily KČT branch. The Krkavčí skála lookout was funded by the Okrašlovací spolek Semily and opened on 18 June 1905.

Two trails run along the right bank: Riegrova stezka and Kamenického stezka. A yellow-marked route follows the left rim, with a branch to Krkavčí skála about 2 km on foot from Semily; drivers can park near the quarry entrance, roughly 200 m from the viewpoint. The climb to Myší skála is secured only partly by chains and needs some climbing technique at the end.

A 12-metre pedestrian tunnel on the Riegrova stezka was blasted through hard volcanic rock in six weeks in 1909, using 1,086 dynamite cartridges. In winter and early spring melt, a roughly 20-metre-wide, 10-metre-high icefall forms near it, covering a rock overhang and enclosing a frozen pool below.