Estimates of long-term suspended-sediment loads in Bay Creek at Nebo, Pike County, Illinois, 1940-80

Five years of daily suspended-sediment discharges (1968, 1969, 1975, 1976, and 1980) for Bay Creek at Nebo, Illinois, computed from once- or twice-weekly samples (more often during storm events), were used to develop transport equations that can be used to estimate long-term suspended-sediment discharges from long-term water-discharge records. Discharge was divided into three groups based on changes in slope on a graph of logarithms of water discharge versus suspended-sediment discharge. Two subgroups were formed within each of the three groups by determining whether the flow was steady or increasing, or was decreasing. Seasonality was accounted for by introducing day of the year in sine and cosine functions. The suspended-sediment load estimated from the equations for the 5 years was 77.3 percent of that computed from daily sediment- and water-discharge records for those years. The mean annual suspended-sediment load for 41 years of estimated loads was 359 ,500 tons, which represents a yield of about 3.5 tons per acre from the Bay Creek drainage basin.