Facilities for passing fish upstream over Lower Monumental Dam include a powerhouse fish collection system, auxiliary water supply systems, and a fish ladder on each side of the river. The main portions of the 16-ft-wide fish ladders are constructed with a floor slope of 1 on 10. The 6-ft-high weirs have two 18- by 18-in. orifices on the floor 3 ft from the fish ladder walls, a 6-ft-long nonoverflow section centered in the ladders, and fins that extend 18 in. upstream from the ends of the nonoverflow sections. The weir crests are like those in the north fish ladder at John Day Dam. Auxiliary water for transportation flow in the ladders and attraction flows at the fishway entrances is pumped through supply conduits and distributed through six diffusion chambers in the floor of each ladder. Flow control for the downstream diffuser is provided by sluice gates from the supply conduit; flow in the other five diffusers is automatically controlled by a weir-and-orifice arrangement in the diffuser walls. A straight, 35-pool section of the south fish ladder was reproduced in a 1:10-scale model. Tests in the model indicated that flow conditions in the typical pools would be satisfactory. Discharges of 66.0 and 69.7 cfs produced heads of 10.0 and 12.0 in., respectively, on the weirs. With 18- by 18-in. orifices in all weirs, heads of 12.2 and 13.4 in. were required to provide the above discharges at the first weir below the fish counting station. Discharge rating data for a typical fishway entrance weir was measured in a 1:10-scale model. The quantity of flow increased with submergence and decreased with weir height. A maximum discharge of 227.5 cfs per ft of channel width was obtained with no weir and 14 ft of submergence. Discharge over a 15-ft-high weir submerged 5 ft was 28.1 cfs per ft of crest length.