Major differences have often been observed between experimental measurement
s and solutions of the one-dimensional dispersion equation for the spreadin
g of tracers in rivers. This paper compares calculated values of peak conce
ntration decay rates, variances, and concentration distributions for three
different one-dimensional models with experimental results obtained by T. J
. Day. The calculations show that the Fickian and dead-zone models have sim
ilar behaviors for peak decay rates and variances, but that these behaviors
do not agree with Day's measurements. The third model, in which the disper
sion coefficient increases linearly with distance downstream gives results
that agree closely with Day's measurements. It is suggested that the increa
se in the dispersion coefficient with distance downstream may be the result
of a relatively large amount of dispersion from velocity shear near the le
ading and trailing edges of the tracer cloud.