This is the larger of the two locks that are on the Rozmberk. It was intended to prevent flooding of the water level above a certain level, overflowing the dam and damaging it from the outside. Wooden fish barriers have also been installed to prevent fish from escaping during high tide. Madness what everything was built here for the flood protection.
Incidentally, the Rozmberk pond is the largest artificial man-made pond in Central Europe! Approximately every 2 years, the pond is emptied to harvest the carp in it. Around 160 (!) Tons of fish are produced in this way.