UTILITY OF USGS STREAMFLOW-GAGING STATION RECORDS TO ASSESS HISTORICAL GEOMORPHIC ADJUSTMENTS ALONG THE LOWER BRAZOS AND SABINE RIVERS, TEXAS AND LOUISIANA, USA

Streamflow-gaging station records of the U.S. Geological Survey (USGS) have commonly been utilized to assess historical geomorphic adjustments to river channels. Historical USGS records include stage-discharge rating curves, hard-copy discharge measurement notes, surveys for indirect peak discharge estimates, ground-based photographs, and construction diagrams. Practitioners using these records, however, must exercise considerable caution during analytical and interpretative phases because of: (i) gaging-station site characteristics (often unique and conducive to minimal alluvial adjustment), (ii) spatial and temporal continuity of measurement section locations, (iii) change to a new arbitrary vertical stage datum, (iv) hydraulic behavior at bridges, (v) bridge construction and repair activities, (vi) limitations of baseflow (low flow) measurements, and (vii) scour-and-fill behavior. An example of the utility of historical USGS records is provided for selected long-term stations along the lower Brazos and lower Sabine Rivers in Texas and Louisiana. Comparisons of historical and contemporary cross sections, rating curves, aerial photographs, and ground-based photographs indicate that: (i) channel incision has occurred along the Sabine River just downstream of Toledo Bend Reservoir, but little overall change is detected at stations more than 50 kilometers downstream, and (ii) accretion of point bars and concomitant channel incision has occurred along the Brazos River since the 1960s, with the greatest incision occurring at the lowermost stations approaching the Gulf of Mexico.