A map of the Ohio River basin above the Muskingum River shows patterns of mean annual runoff for the new climatologic and hydrologic reference period, 1941-70, and provides an up-to-date, consistent basis for consideration of this streamflow characteristic. The primary data base consisted of 98 long-term gaging-station records collected within this 27,300-square-mile (70,700-square-kilometer) headwater area of the Ohio River basin. Supplemental information was derived from 83 short-term records.
Mean annual runoff is at a regional minimum of less than 12 inches (300 millimeters) in an area extending from the northern West Virginia panhandle to the headwaters of the Mahoning River in Ohio. Mean annual runoff of more than 32 inches {810 millimeters) occurs in parts of the upper Cheat River basin in West Virginia. The zone of high runoff trends northeastward along the western slopes of tile Allegheny Mountains; magnitudes diminish to about 25 inches (630 millimeters) at the eastern basin boundary near Ebensburg, Pa. Runoff of this magnitude occurs also in a band across tile upper Allegheny River basin in Pennsylvania.
Ratios of mean annual discharge to the 30-year reference-period average were computed for each year of record for all long-term gaging stations. Annual discharges have ranged from about 30 percent to 200 percent of the 30-year mean. Graphed summaries of the annual ratios document the relative duration and magnitude of wet and dry periods during the past 65 years in the upper Ohio River basin.
