ON EFFECTIVE DISCHARGE AND CHANNEL-FORMING STREAMFLOWS

Effective discharge has been defined as the increment of discharge transporting the largest fraction of the annual sediment load over a period of years, and a channel maintenance flow as a discharge that transports the sediment inflow over the long term with neither aggradations nor degradation. Common knowledge is that effective and channel-maintenance discharges are the same. Suspended sediment data for the Klamath River at Orleans, California was used to calculate the effective discharge based on three alternative assumptions. The equation for sediment concentration, C_s, used in the analysis was C_s= a(Q-Qcrt)^b-1. The first alternative is to use total suspended sediment concentrations. The critical discharge, Qcrt, is zero. The power term, b-1, was determined by regression to be 1.39 and the effective discharge calculated to be 670 m³/s. The second alternative was the transport of sand. For sand b-1 is 0.66 with a Qcrt of 350 m³/s. The effective discharge for sand is also 670 m³/s. The power term for gravel is not known; a review of the literature indicates the minimum value is 4.0. The critical discharge for gravel was estimated from hydraulic analysis to be 930 m³/s. The effective discharge for gravel/cobble movement is .6710 m³/s (essentially the maximum discharge in the streamflow record). All three are probably good estimates of the effective discharge to move different sizes of sediment. The total sediment and sand effective discharge is smaller than the minimum discharge required to move the substrate and is not the discharge required to maintain the channel because the substrate most be moved in order to maintain the channel. The maximum discharge in the 82 years of record is not likely to be a good estimate of the channel maintenance discharge because cobbles and gravels are deposited in the channel by tributaries and mass movements more frequently than once in 82 years. An alternative is to consider the discharge with a 1 % probability of moving the substrate (1548 m³/s) as the channel maintenance discharge. The channel maintenance discharge occurs in 45% of the years (return period of 1 in 2.2 years). Effective discharge and channel maintenance discharges are not the same in a gravel/cobble-bed river.