Defining a Riverine Tidal Freshwater Zone and Its Spatiotemporal Dynamics

Tidal freshwater zones (TFZs) are transitional environments between terrestrial and coastal waters. TFZs have freshwater chemistry and tidal physics, and yet are neither river nor estuary based on classic definitions. Such zones have been occasionally discussed in the literature but lack a consistent nomenclature and framework for study. This work proposes a measurable definition for TFZs based on three longitudinal points of interest: (1) the upstream limit of brackish water, (2) the upstream limit of bidirectional tidal velocities, and (3) the upstream limit of tidal stage fluctuations. The resulting size and position of a TFZ is transient and depends on the balance of tidal and riverine forces that evolves over event, tidal, seasonal, and annual (or longer) timescales. The concept, definition, and transient analysis of TFZ position are illustrated using field observations from the Aransas River (Texas, USA) from July 2015 to July 2016. The median Aransas TFZ length was 59.9 km, with a late summer maximum of 66.0 km and a winter minimum of 53.6 km. The TFZ typically (annual median) began 11.8 km upstream from the river mouth (15.4 km winter/11.2 km summer medians) and ended 71.7 km upstream (69.0 km/77.2 km). Seasonally low baseflow in the Aransas River promoted gradual coastal salt encroachment upstream, which shortened the TFZ. However, sporadic large rainfall/runoff events rapidly elongated the TFZ. The TFZ definition establishes a quantifiable framework for analyzing these critical freshwater systems that reside at the nexus of natural and human-influenced hydrology, tides, and climate. Plain Language Summary Rivers reaching the coast begin to feel the impact of tides. Coastal river segments can rise and fall with the tide and even flow inland at certain times during the tidal cycle. At the same time, the tide can push salty coastal water upstream, traveling from offshore up the river channel. This study focuses on the river segment between that salty water and freshwater flowing toward the coast. This region of the river is the tidal freshwater zone, (TFZ) because this river water is both fresh, meaning it contains effectively no salt, and tidal, since it moves with the tide. In this paper, we define the key characteristics of a TFZ, and observe one of these zones in a coastal river in Texas over 1 year. We witnessed the TFZ expand and shrink like an accordion as rain events added more freshwater, and droughts removed freshwater allowing the tide to push salt water upstream quickly. We also observed the zone move back and forth along the river channel as the seasons and climate changed. Future coastal water management plans should account for the TFZ since this river segment may move vital nutrients to the coast very slowly. Also, climate change may strongly impact TFZs, since these zones are influenced by two major consequences of climate change: rising sea level (i.e., tide) and rising temperatures (i.e., altered water cycle). Key Points Established measurable definition of tidal freshwater zone (TFZ) and observed and analyzed tidal freshwater zone in Texas, USA Median zone length was 59.9 km in Aransas River, TX, USA, with late summer maximum length of 66.0 km and a winter minimum of 53.6 km Runoff events lengthened and seasonal drought shortened the zone, which was also mobile along channel due to changing tide and climate