Water level records from two study sites in Indian River Lagoon, along Florida’s Atlantic Coast, are used to characterize the vertical displacement of the estuarine intertidal zone in response to subtidal frequency forcing. A 22-year water level record indicates that the seasonal cycle has a range approximately one-quarter greater than the mean tidal range. The intertidal zone thus rises and falls to such an extent that over time scales in excess of several weeks there is no layer which consistently experiences an alternating exposure and inundation. Six-year sets of high and low tide extremes from the second study site are expressed in the form of cummulative histograms to determine the probabilities with which high tide and low tide levels lying outside of median values will occur in response to the interaction of tidal constituents and low-frequency forcing. High and low water values are then stratified by month, and probability distributions are recomputed for each subset. In this study area, unpredictable, low-frequency water level fluctuations perturb the intertidal zone to such an extent that the probabilities of extreme high and low water levels, in addition to mean high and low water, must be determined to characterize the stuarine intertidal zone adequately. *** DIRECT SUPPORT *** A01BY034 00002