CHANGES IN SEDIMENT TRANSPORT CAPACITY: COLORADO PLATEAU AND GREAT BASIN

An index to the capacity of streams to transport sediment has been use in the analysis of environmental streamflows in a number of studies. Daily streamflows are used to calculate a sediment transport capacity index (stci) for each day. The equation is stci=(q*(qƒ{qcrt)^b)/(qref^(b+1)) where q is the daily streamflow, qcrt is a critical discharge, b is a power term based on the relation between discharge and the sediment concentration (default value of 2.0 if a relation is not available), and qref is a reference discharge. An annual STCI is the sum of the daily values for a water year. For the Humboldt River at Palisade, Nevada the ratio of annual discharge for the second half of a 99 year period of record divided by the first half is 1.22 annual discharge, and 1.29 for the maximum annual daily discharge compared to1.60 for the STCI. The STCI integrates the impact of peak streamflows and the duration of the streamflows. Changes in the STCI tend to reflect changes through time much more clearly than either the annual flows or maximum annual daily flows. The topic of this paper is the variation of the annual STCI between years. Few long record gaging stations exist in the Colorado Plateau and the Great Basin. These few show a decrease in STCI after 1922-23 compared to the previous years in the twentieth-century. Change to a higher STCI occurred in 1941-42 in parts of the region but later in other parts. Two years are an anomaly (1983 and 1984) in the whole of the region in that the STCI is significantly larger (for the Humboldt River station the 1984 STCI was 2730 compared to the average for 1912-2007 of 228). There is also much more variation in the STCI following about 1977 compared to previous periods. The relation between the STCI and sediment load may have changed around 1944 at Green River, Utah on the Green River. In the period 1930-1944 the annual load at an STCI of 300 was 100 kilotons. This changed to 48 kilotons in 1945-1962.