Where the river meets the sea : an initial investigation into the Riverine tidal freshwater zone

Riverine tidal freshwater zones (RTFZs) are transitional environments between terrestrial and coastal waters that have freshwater chemistry and tidal physics, and are neither river nor estuary. The residence time dynamics of RTFZs have not yet been discussed in the literature, but may ultimately control the timing and magnitude of freshwater and nutrient coastal discharges. Furthermore, climate change threatens to alter the nexus between the terrestrial hydrologic cycle and the coastal tidal environment where the RTFZ resides. This dissertation provides the foundation for investigations into RTFZs and their subsequent residence times.

An initial residence time analysis of 15 tidal river reaches along the south Texas Gulf Coast introduces tidal river reaches into the lentic/lotic nomenclature. This residence time analysis also quantifies the vernacular of lentic (reservoir-like) and lotic (riverine) systems via the Freshwater Continuum Classification (FCC). The FCC framework also incorporates temporal hydrologic variability, which is typically absent from other lentic/lotic classifications.

Further analysis on one of these systems (Aransas River, TX, USA) revealed an RTFZ. The analysis empirically observed RTFZ responses to precipitation and tide, while providing the RTFZ definition. The RTFZ is defined by three longitudinal points of interest: λ₁ – upstream limit of brackish water, downstream limit of freshwater, and downstream boundary of RTFZ; λ₂ – upstream limit of bidirectional tidal velocities; λ₃ – upstream limit of tidal stage fluctuations and of the RTFZ. The RTFZ was typically (median) 59.9 km long and typically (median) began 11.84 km upstream (15.43 km/11.16 km, max/min) of the river mouth.

From field data collected during RTFZ isolation, a tidal rating curve was created to model tidal discharge. The tidal rating curve expanded on traditional non-tidal gauging methods by incorporating stage-rate-of-change observations into the typical stage to discharge relationship. The method was performed on several sites along the Mission and Aransas Rivers, TX, USA, and twelve USGS tidal gauging stations, and resulted in strong agreement between estimated and observed discharge (i.e., r² > 0.70). With the tidal discharge estimations and RTFZ observations, future investigations should focus on RTFZ residence times and their impacts on estuarine ecology.