Defining surface water systems as lentic or lotic is an important first step in linking hydrology and ecology. Existing approaches for classifying surface water as lentic (reservoir‐like) or lotic (river‐like) use qualitative observations, solitary snapshot measurements in time and space, or ecologic metrics that are not broadly repeatable. This study introduces the Freshwater Continuum Classification (FCC), a quantitative method to consistently and objectively classify lentic/lotic systems based on integrated residence time (iTR), the time incoming water would take to exit the system given observed temporal variations in the system's discharge and volume. Lentic/lotic classification is determined from comparison of median iTR with critical flow thresholds related to key time scales such as zooplankton generation. Some systems switch between lentic and lotic behaviors over time, which are additionally defined in the FCC as oscillic. Pilot application of the FCC to 15 tidally influenced river segments along the Texas Gulf Coast produced good agreement with previous methods of determining lentic/lotic character. The FCC defined 8 of 15 tidal reaches as primarily lentic, 6 as intermediate, and 1 as lotic between October 2007 and March 2015. Of the 15 reaches, 9 were also oscillic, characterized in this climate by short‐lived lotic character during flash floods. The FCC provides a broadly applicable, repeatable, quantitative method to classify surface water bodies as lentic/intermediate/lotic and oscillic/nonoscillic regardless of size or nature (e.g., river or reservoir) based on system volume and flow characteristics.

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).

Empirically quantifying tidally-influenced river discharge is typically laborious, expensive, and subject to more uncertainty than estimation of upstream river discharge. The tidal stage-discharge relationship is not monotonic nor necessarily single-valued, so conventional stage-based river rating curves fail in the tidal zone. Herein, we propose an expanded rating curve method incorporating stage-rate-of-change to estimate river discharge under tidal influences across progressive, mixed, and standing waves. This simple and inexpensive method requires (1) stage from a pressure transducer, (2) flow direction from a tilt current meter, and (3) a series of ADP surveys at different flow rates for model calibration. The method was validated using excerpts from 12 tidal USGS gauging stations during baseflow conditions. USGS gauging stations model discharge using a different more complex and expensive method. Comparison of new and previous models resulted in good R2 correlations (min 0.62, mean 0.87 with S.D. 0.10, max 0.97). The method for modeling tidally-influenced discharge during baseflow conditions was applied de novo to eight intertidal stations in the Mission and Aransas Rivers, Texas, USA. In these same rivers, the model was further expanded to identify and estimate tidally-influenced stormflow discharges. The Mission and Aransas examples illustrated the potential scientific and management utility of the applied tidal rating curve method for isolating transient tidal influences and quantifying baseflow and storm discharges to sensitive coastal waters.

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