In this paper we consider decentralised PI control of the water levels in an irrigation channel. The water levels are controlled using overshot gates located along the channel, and the output of the controllers is the head over gate. This choice of manipulated variable is advantageous since the head over gate is directly related to the flow over the gate. The PI controllers are augmented with a first order low pass filter in order to ensure a low gain at the resonant wave frequency. A distant downstream controller configuration with feedforward is used. In this configuration a gate controls the water level immediately upstream of the next downstream gate. The controllers are tuned based on simple linear system identification models using frequency response methods, and the designed controllers have shown very good performance in field tests. The water levels recovered smoothly from disturbances without excessive oscillations, and the deviations from setpoints were small.