In order to reduce the recovery time of a given water level in front of the proposed automatic water retention device in a trapezoidal channel, a theoretical formula was obtained for determining the recovery time. And based on experimental studies, a simpler formula was obtained for determining the time to restore a given water level in the channel in front of the installation site of this device. Experimental studies to obtain this formula were conducted in accordance with the criteria of Froude similarity and dynamic similarity of forces. This formula is consistent with the theoretical one and shows that reducing the recovery time of a given water level in front of the proposed automatic water retention device is affected by an increase in the area of the outlet openings from the shut-off tank of the automatic water retention device. A shorter time interval for restoring the water level makes it possible to achieve the required accuracy in supplying the required water flow to the water outlet devices located on the trapezoidal channel in front of the automatic water retention device. And also to improve the infrastructure of the canal due to more rapid regulation of the water level on the trapezoidal channels.