Runo} characteristics in a low!gradient central!Florida watershed were analysed using environmental d 07 O and a series of conservative!tracer mass!balance models applied to a storm event "098 mm over approximately 29 h# monitored in May and June 0882 on the Econlockhatchee River\ Florida[ The assumption of steady! state conditions implicit in the widely used two!component mixing model was tested by applying steady!state and non!steady!state models for a subcatchment "104 km 1 # of the river[ Both models indicated that about 65) of the resulting river ~ow was composed of pre!storm water[ A third mass!balance model "steady!state# was developed to separate pre!storm from storm!event runo} over a discrete reach of the river\ which had a contributing area of 024 km 1 [ This model indicated that approximately 36) of the water entering the reach could be attributed to pre!storm water[ The greater proportion of event water entering the reach was attributed to suburban development in the watershed and indicates that urbanization in watersheds not only a}ects the timing\ peak and total runo}\ but also may change ~ow!paths for runo}\ and may signi_cantly a}ect down! stream water quality [ Copyright Þ 1999