Paleoflood analysis along the North Concho and Guadalupe Rivers, central Texas; improving riverine flood frequency estimates using paleoflood data and comparisons to July 2025 extreme flooding in Kerrville, Texas

Two paleoflood analysis (PFA) reduced uncertainty in the perception of the frequency of extreme flooding along the North Concho and Guadalupe Rivers in central Texas. In early 2025, significant flood stages were experienced on the upper Guadalupe River near Kerrville, Texas. High water mark survey and field observation data of fluvial deposits were collected during post-2025 flood analysis efforts along the Guadalupe River. Flood-plains and discontinuous terrace remnants are ideal landforms for preserving large magnitude flood deposits in these analyses. Pre-historic flood (paleostage indicators, PSI) and non-inundation features (non-exceedance bounds, NEB) were identified via geomorphic mapping using high resolution digital elevation models and field observations. One- and two-dimensional hydraulic models were utilized for discharge estimation of the paleoflood features. The paleoflood chronology is interpreted from relative soil development on sandy deposits associated with floodplains and terrace remnants with samples dated by optically stimulated luminescence and 14C dating. Geologic evidence suggests that the 1936 flood of record on the North Concho River was the largest flood to have occurred within the last 370 years, based on the presence of eolian deposits that demonstrate an absence of flood inundation. The elevations of these non-fluvial sands represent non-inundation limits to flood discharges of about 210,000 cfs and 260,000 cfs. The Guadalupe River valley holds geologic evidence of at least two PSI underlying correlated fluvial terrace remnant surfaces: one historic flood dated to approximately 1869CE with a peak discharge estimate of 220,000 cfs, and an older, late Holocene paleoflood that occurred approximately 1,600 years ago (peak discharge of approximately 280,000 cfs). Data from July 2025 extreme flooding serve as a modern analog for benchmarking the interpretation of flood discharges for the paleoflood features identified along the Guadalupe and North Concho Rivers.