Relation of sediment load and flood-plain formation to climatic variability, Paria River drainage basin, Utah and Arizona
Suspended-sediment load, flow volume, and flood characteristics of the Paria River were analyzed to determine their relation to climate and flood-plain alluviation between 1923 and 1986. Flood-plain alluviation began about 1940 at a time of decreasing magnitude and frequency of floods in winter, summer, and fall. No floods with stages high enough to inundate the flood plain have occurred since 1980, and thus no flood-plain alluviation has occurred since then. The decrease in magnitude and frequency of floods appears to have resulted from a decrease in frequency of large storms, particularly dissipating tropical cyclones, and not from a decrease in annual or seasonal precipitation.

Suspended-sediment load is highest in summer and fall, whereas flow volume is highest in winter. Fall shows the greatest interannual variability in suspended-sediment load, flow volume, and flood size because climatic conditions are most variable in fall. The relation between sediment load and discharge apparently did not change within the period of sediment sampling (1949-1976), even though the channel elevation and width changed significantly. Annual suspended-sediment loads estimated for periods before and after 1949-1976 show that decrease in suspended-sediment load caused by floodplain alluviation in the Paria River and other tributaries could have been a significant part of the decrease of suspended-sediment load in the Colorado River in the early 1940s.