Facilities for passing fish upstream over Lower Monumental Dam include a powerhouse collection system and a 16-ft-wide fish ladder on both sides of the river. A straight, 35-pool section of fish ladder and a typical fishway entrance weir were reproduced in a 1:10-scale model. Performance of a pair of typical diffusion chambers was studied in a 1:8-scale model that included portions of the adjacent supply conduit and fish ladder. Fishway weirs of original design, with 5-ft-long overflow crests at each end of a 6-ft-long nonoverflow section, upstream fins, and 18- by 18-in. orifices on the floor, were satisfactory. Discharges of 66.0 and 69.7 cfs produced heads of 10.0 and 12.0 in. on the weirs. With standard orifices in all weirs, heads of 12.2 and 13.4 in. on the first weir below the fish counting station were required to provide the above discharges. (Modified author abstract).