Parshall flumes are commonly used devices for the measurement of flow rates in open channels such as sewers and irrigation ditches. The operation of these flumes in "nonstandard" situations, such as when faced with distorted upstream velocity profiles or when placed in sloping channel beds, is poorly understood. This paper presents numerical solutions for the flow through Parshall flumes utilizing a three-dimensional, free-surface finite difference code. The effects on flume operation of channel slope, upstream velocity profile distortions, and flume geometry changes are assessed. Experimental results are obtained for a Parshall flume with a 15.2 cm (6 in.) throat width. Experimental and numerical data on important features of the flume flowfield are seen to compare well.