Douglas J. Parkinson Andrew J. Miller 1993 <p>The November 1985 flood was the largest recorded in the South Branch Potomac River basin. Discharges&nbsp;exceeded values estimated for a recurrence interval of 500 yr at four of six stations in the basin. Flow velocities in the&nbsp;channel were as high as 4.6 m/s and may have exceeded 6&nbsp;m/s at some locations; estimated values of unit stream&nbsp;power at U.S. Geological Survey gage locations were as&nbsp;high as 988 W/m2 and may have exceeded 2,500 W/m² at&nbsp;some locations. Along the three forks of the South Branch&nbsp;Potomac River, discharges of 1,000-7,000 m³/s flowed&nbsp;through valley cross sections that ranged in width from&nbsp;less than 70 m to 1,700 m, and, as a result, hydraulic&nbsp;conditions were extremely variable. Valley constrictions&nbsp;and expansions were important determinants of erosion&nbsp;and deposition patterns.&nbsp;Geomorphic impacts on valley floors along the three&nbsp;forks of the South Branch Potomac River included formation of a wide array of erosion and deposition features.<br>Flood-generated erosion features included longitudinal&nbsp;grooves, scour marks, widened channels, stripped flood&nbsp;plains, chutes, anastomosing erosion channels, and jetshaped erosion forms. Deposition features were generally&nbsp;adjacent to eroded areas and included channel gravel&nbsp;bars, gravel splays, gravel and sand sheets, isolated gravel&nbsp;bars and sand dunes on flood plains, wake deposits, and&nbsp;backwater deposits.&nbsp;Mapping of erosion damage classes along 384 km of&nbsp;channel and valley floor revealed that 82.8 percent of the&nbsp;valley length experienced at least incipient erosion, 30.2&nbsp;percent experienced severe erosion, and 5.7 percent&nbsp;experienced catastrophic erosion of the valley floor.&nbsp;Reach-average values of unit stream power were positively&nbsp;correlated with the percent of the valley reach that experienced severe erosion; percentages of 50 percent or&nbsp;more were associated with average values of 200-500&nbsp;W/m². These results suggest that, with further research, it&nbsp;should be possible to define threshold conditions for&nbsp;bottomland erosion.&nbsp;</p> application/pdf 10.3133/b1981E en U.S. Geological Survey Flood hydrology and geomorphic effects on river channels and flood plains: The flood of November 4-5, 1985, in the South Branch Potomac River Basin of West Virginia reports