The nonlinear distortion of the astronomic tide in the Lower St. Johns River is investigated. Computed tidal elevations are analyzed at various locations within the Lower St. Johns River. The modeling approach first evaluates the boundary condition applied at the open ocean with regards to it providing a complete description of the tidal elevation, followed by numerical experimentation and a tidal constituent analysis that examines the effects of finite wave amplitude, advection, and bottom friction towards distorting the tide as it propagates upriver. The distortions caused by each nonlinear source are presented in both the time and frequency domains. Analysis at observation stations reveals a river tide with more coastal characteristics near the mouth and with more considerable distortion upriver. The spectroscopy of the astronomic tide for the Lower St. Johns River is established in terms of a custom set of tidal constituents.