Various models used to describe or predict longitudinal mixing coefficients are examined and the conditions under which they are expected to apply are elucidated. Several hundred coefficients, both in laboratory flumes and natural streams, were measured under two-dimensional, uniform flow conditions. The coefficients measured in natural streams are shown to follow equations derived from flume data. The influence of dead zones in the stream on mixing is examined, and it is shown that measured coefficients are much closer to predicted values when the influence of mixing due to the dead zones is separated by the use of a new mixing model.