Monongahela Darn 2, located 11.2 miles upstream of the confluence of the Ohio, Allegheny, and Monongahela Rivers, in North Braddock, PA, maintains the l2.6-mile-long navigation pool between the Dam 2 and Dam 3 locks and dams. The U.S. Army Engineer District, Pittsburgh, developed a 'two-for-three' plan for renovating locks and dams on the lower Monongahela River that would save the cost of having to reconstruct Lock and Darn (L & D) 3 and reduce transportation costs by eliminating bottlenecks caused by the small locks at L & D's 3 and 4 and by reducing one lockage cycle. The plan calls for building a new gated dam at the current L & D 2, eliminating lAD 3, and replacing the locks at L & D 4 with new, larger locks. The change would also mean Pool 2 would be raised by about 5 ft, and the current Pool 3 would be lowered by about 3 ft. The darn now proposed for the lAD 2 site will consist of a navigable gated structure with three low sills (crest el 704.7), a water quality sill (crest el 714.0), and a fixed-crest weir (crest el 723.7). (All elevations (el) are given in feet referred to the National Geodetic Vertical Datum). The spillway sectional model was conducted to investigate the hydraulic performance to be expected with the low sills and water quality sill under long-range operating conditions for controlled and uncontrolled flows. The model study provided the data necessary to evaluate and develop a satisfactory means of regulating the structure to achieve the desired flow objectives without creating adverse hydraulic conditions. (MM).