The invention relates to a maximum water level gauge for a river channel. The maximum water level gauge comprises a cylindrical cylinder, a one-way paying-off mechanism fixedly arranged on the bottomof the cylinder and a floating ball arranged in the cylinder, wherein the diameter of the floating ball is less than the inner diameter of the cylinder, the lower portion of the cylinder is provided with a plurality of water inlets, the outer portions of the water inlets are provided with a protection device, the one-way paying-off mechanism comprises a shell body, a rotating shaft arranged in theshell body and a first coiling wheel fixedly arranged on the rotating shaft, the two ends of the rotating shaft are connected with the shell body through bearings, the rotating shaft is further fixedly provided with a ratchet wheel, the shell body is internally provided with a detent adaptive to the ratchet wheel, the first coiling wheel is provided with a first pull line, the top end of the first pull line is connected with the bottom of the floating ball, and the top of the floating ball is connected with the top of the cylinder through a connecting line. The maximum water level gauge for the river channel has the advantages that the floating ball is adopted to pull the pull line to record so as to reduce resistance and errors, thereby ensuring that the floating ball can float steadilyalong with the water level of the river channel, a floating component adopts the floating ball, the diameter of the floating component is less than the inner diameter of the cylinder, and a gap is reserved to ensure that the floating ball can rise freely without the resistance of the cylinder.