This paper presents the results from a physical model study of a fixed crest ogee spillway with lateral inflow creating a contraction loss at the inlet reducing the effective spillway width. The model represents a scenario faced by some spillway structures in low head run-of-river hydropower schemes. The contraction caused at the corner of the spillway inlet is quantified for varying discharges and its influence on the spillway capacity is evaluated. To improve the capacity of the spillway, firstly the approach angle is altered. The spillway capacity is measured for multiple headwall angles, finding a relative increase of 6.2 % in discharge capacity for a 55° approach. Secondly, guide walls of varying designs in the direction of the lateral flow are introduced. The design of the modified guide wall seeks to smoothen the approach and reduces the ensuing flow separation, thus increasing the capacity of the spillway. The greatest improvement is found for an elliptic guide wall which increased the discharge capacity by 6.7 %. Ultimately, small structural changes can make significant improvements to spillway capacity.