Secular changes in the tidal constituents in San Francisco Bay originated by the California Gold Rush and major dam-building projects

Abstract:

Hourly sea-level records for the time period of 1901 to 2012 at Fort Point tidal station in San Francisco Bay are analyzed in an attempt to find the origin of the secular changes found in the tidal constituents. Complex demodulation implemented with a low pass filter window of 8760 hours was employed to extract the amplitude and phase of the principal tidal constituent M₂ as a function of time. The 18.6 year nodal signal was removed by using the tide potential of the equilibrium tide. The results show significant trends up to 4 cm in amplitude as well as phase shifts of 17 minutes per century. Moreover, yearly amplitude variations of M₂ show to be inversely correlated to river flow discharge. On the other hand, the results of a simplified two-layer numerical hydrodynamic model indicate that long-term tide variations are directly related to the morphological evolution of a sandbank located outside the bay surrounding its entrance, acting as a hydraulic control for the whole bay. According to historical results, the sandbank reached its shallowest depth during the California Gold Rush (1848-1884), when mining debris together with large amounts of sediment were deposited into the estuary. After the Central Valley Water Project was approved (1933), many dams were built decreasing significantly the sediment supply. With the passage of time, the gradual loss of sedimentation also diminished the sandbank, increasing its depth. This fact explains the observed secular long-term advance of the tidal phase, as well as the increasing trend of the M₂ amplitude.