Annual streamflows from the 21 water-resource regions in the United States and Puerto Rico were calculated for the period 1951-83. The total streamflow discharging to the oceans from the conterminous United States during this period averaged 1,270 billion gallons per day. The outflow from the Lower Mississippi Water-Resource Region (08), which drains 41 percent of the land area of the conterminous United States, contributes 34 percent of the total streamflow to the oceans, which is the most of any region.
A 15-year moving average of the annual streamflow was also calculated for each region. The Mid-Atlantic (Region 02) shows a decreasing trend in streamflow since the mid-1970's whereas the Upper Mississippi (Region 07), the Missouri (Region 10), the Rio Grande (Region 13), the Upper Colorado (Region 14), the Lower Colorado (Region 15), the Great Basin (Region 16), and California (Region 18) all show an increasing trend in streamflows.
Streamflow from the Upper and Lower Colorado (Regions 14 and 15) appear to be heavily affected by large storage reservoirs. Streamflow from the Upper Colorado (Region 14) shows a decreasing streamflow in 1963 and 1964, which may be due to filling of Lake Powell.
