Depositional aspects of the November 1985 Flood on Cheat River and Black Fork, West Virginia

Widespread, intense rainfall in November 1985 produced floods that exceeded all historic events on Cheat River and most of its tributaries. Official discharge estimates for Cheat River ranged from 4,800 to 5,380 m³ /s (170,000-190,000 ft³/s) with a recurrence interval of >100 yr. In addition to considerable property damage and the loss of five lives, the November 1985 flood left a variety of deposits, many of which differ from those produced by moderate floods. 

Clay or silt deposits were uncommon after the flood; most were restricted to slack-water deposits at the mouths of hydraulically dammed or back-flooded tributaries. These slack-water deposits were composed of four different sedimentary units: basal gravel and sand (unit A), sandy loam (unit B), silt loam (unit C), and upper fine sandy loam (unit D). The four units always occurred in the same stratigraphic order, but low-gradient tributaries lacked the top and bottom units. The silt loam unit was deposited by water from Cheat River, but the other three were derived from the tributaries. Simple one-unit sandy slack-water deposits formed near very large boulders and bridges. Neither type of slack-water deposit proved to be an accurate indicator of flood stage, so these deposits may be imprecise paleohydraulic indicators for central Appalachian streams. 

Sand dunes and arcuate splays of cobbles and small boulders developed on flood-plain sites downstream from isolated scours or erosional ramps attached to the river channel. Gravel deposits commonly were juxtaposed on top of sandy preflood alluvium, providing evidence of considerable tractive force in what is normally a low energy overbank environment. The flood transported large volumes of cobbles and boulders from the channel margin to sites hundreds of meters from the channel. Most of the mobilized sediment originated on the flood plain.

Extremely large boulders (>2.75-m intermediate axes) were transported in two of the steeper reaches of Cheat River. Published empirical equations relating stream competence to mean flow velocity, tractive force, and unit stream power suggest these boulders approached the largest size that a flood of this magnitude could transport. 

Trees and trash appeared to be the most voluminous sediments left by the November 1985 flood. These nonclastic deposits commonly were scattered widely about the flood plain, but they also occurred as clusters of dunelike forms on unforested surfaces or as thick lobate forms on forested bottomlands. 

Postflood mitigation has destroyed most of the November 1985 flood deposits, precluding detailed study of some effects of the flood. If the extensive mitigation had not occurred, some of the morphologic and stratigraphic effects of this extreme flood would have persisted in the Cheat River and Black Fork fluvial systems for centuries.