These data were recorded a 5 monitoring sites along each of the Mission and Aransas Rivers of south Texas, USA. Installed instruments observed temperature, conductivity, water depth, and tidal velocity from May 2015 - July 2017. The monitoring sites observed a riverine tidal freshwater zone (RTFZ) in each river. We define an RTFZ as a river reach composed of freshwater chemistry and impacted by tidal physics (i.e., bidirectional velocity and/or surface water oscillations), which is bounded upstream by unidirectional fresh riverine discharge and downstream by an estuarine brackish water column, all of which is upstream of the river mouth. The size and position of an RTFZ is transient and depends on the balance of tidal and riverine forces that evolves over event, tidal, seasonal, and annual (or longer) timescales.

Between July 2016 and July 2017, the median Aransas RTFZ length was 59.90 km, with a late summer maximum of 66.02 km and a winter minimum of 53.58 km. The RTFZ typically (annual median) began 11.84 km upstream from the river mouth (15.43 km winter/11.16 km summer medians) and ended 71.74 km upstream (69.01 km /77.18 km). Seasonally low baseflow in the Aransas River promoted gradual coastal salt encroachment upstream, which shortened the RTFZ. However, sporadic large rainfall/runoff events rapidly enlarged the RTFZ.