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