This manual describes how to design, construct, and operate long-throated flumes to measure and control water flow in open channels. Broad-crested weirs with a streamlined flow contraction are members of the long-throated flume family and can be analyzed with the techniques described herein. Broad-crested weirs are particularly well adapted to irrigation canals, while long-throated flumes are well adapted to natural streams. The range of conditions over which flow can be measured with long-throated flumes and broad-crested weirs is unlimited. A large variety of examples are presented. This manual includes a revised mathematical model for designing a flow-measuring structure in any open channel to satisfy user-specified hydraulic boundary conditions and design requirements. Upon completion of the design, the model will compute the head-versus-discharge relation and the hydraulic energy losses for the flume or weir. The computer program presented in this publication can accommodate a wide variety of structure and channel shapes as well as many different input and output units. This version greatly expands upon the capabilities of previously published programs.