Measurements of longitudinal dispersion, transverse mixing, channel geometry, and transverse velocity distribution were made in the Missouri River at a flow of about 33,000 cubic feet per second. The results show that the longitudinal dispersion coefficient for the 141-mile reach from Sioux City, Iowa, to Plattsmouth, Nebr., is about 16,000 square feet per second (approximately 5,600 U*d, where U* is the shear velocity and d is mean depth). The transverse mixing coefficient, Ez, for a 6-mile reach immediately downstream from Blair, Nebr., is about 1.3 square feet per second (approximately 0.6 U*d). The value of the longitudinal dispersion coefficient is one of the largest ever measured, and the value of the ratio Ez/U*d is approximately three times as large as that frequently reported for small straight channels.