The annual runoff in the Nation’s rivers and streams during water year 2021 (9.43 inches) was almost the same as the long-term (1930–2021) mean annual runoff of 9.42 inches for the contiguous United States (fig. 1). Nationwide, the 2021 annual average streamflow ranked the 46th highest out of the 92 years.

Figure 1. Annual runoff in the contiguous United States, water years 1930–2021.

Annual mean streamflow also was ranked by State, which in this summary includes the District of Columbia, and Puerto Rico combined with the Virgin Islands. Streamflow was much below normal in California, Colorado, Idaho, Nevada, New Mexico, North Dakota, Utah, and Wyoming (fig. 2). Streamflow was below normal in Alaska, Arizona, Hawaii, Maine, Minnesota, Oregon, and Vermont. Streamflow was above normal in Alabama, Arkansas, Connecticut, the District of Columbia, Florida, Kentucky, Maryland, Mississippi, New Jersey, and Tennessee. Streamflow was much above normal in Delaware, Louisiana, North Carolina, and Virginia.

Figure 2. Statewide streamflow ranks of the United States for water year 2021 compared to mean annual streamflows for water years 1930–2021.[Some colors in the explanation may not be used on the map; **, for Puerto Rico and the Virgin Islands, 78 years of available data were used and the rank was adjusted accordingly]

The United States, the District of Columbia, and Puerto Rico combined with the Virgin Islands lie in 21 large drainages, or water-resources regions (fig. 3). These water-resources regions are based on surface topography and contain the drainage area of a major river; the combined drainage areas of a series of rivers, such as the Texas-Gulf region, which includes several rivers draining into the Gulf of Mexico; or the area of an island or island group. Water-resources regions provide a coherent, drainage basin based framework for depicting streamflow variations.

Figure 3. Water-resources regions of the United States.

Annual mean streamflow was ranked lowest in the Rio Grande region (fig. 4). Annual mean streamflow was ranked much below normal in the California, Great Basin, and Upper Colorado regions. Annual mean streamflow was ranked below normal in the Alaska, Hawaii, Lower Colorado, Pacific Northwest, and Souris-Red-Rainy regions. Annual mean streamflow was ranked above normal in the Arkansas-White-Red, Lower Mississippi, South Atlantic-Gulf, Tennessee, and Texas-Gulf regions.

Figure 4. Regional streamflow ranks in the United States for water year 2021 compared to mean annual streamflows for water years 1930–2021.[Some colors in the explanation may not be used on the map; **, for Puerto Rico and the Virgin Islands, 78 years of available data were used and the rank was adjusted accordingly]

Autumn (October–December 2020) streamflow was ranked lowest in Arizona (fig. 5). Streamflow was ranked much below normal in Colorado, Hawaii, New Mexico, and Utah. Streamflow was ranked below normal in California. Streamflow was ranked above normal in Florida, Maine, Mississippi, New Jersey, South Carolina, South Dakota, and Wisconsin. Streamflow was ranked much above normal in Delaware, the District of Columbia, Louisiana, Maryland, North Carolina, and Virginia. Nationwide, autumn-season streamflow ranked the 33d highest out of 92 years.

Figure 5. Autumn (October–December 2020) statewide streamflow ranks in the United States compared to mean quarterly streamflows for water years 1930–2021.[Some colors in the explanation may not be used on the map; **, for Puerto Rico and the Virgin Islands, 78 years of available data were used and the rank was adjusted accordingly]

Winter (January–March 2021) streamflow was lowest in Arizona (fig. 6). Streamflow was ranked much below normal in California, Colorado, Nevada, New Mexico, and Utah. Streamflow was ranked below normal in Idaho, New York, North Dakota, Pennsylvania, Puerto Rico and the Virgin Islands, and Rhode Island. Streamflow was ranked above normal in Delaware, Kansas, Kentucky, Missouri, Nebraska, South Carolina, Wisconsin, and Virginia. Streamflow was ranked much above normal in North Carolina. Nationwide, winter-season streamflow ranked the 47th highest out of 92 years.

Figure 6. Winter (January–March 2021) statewide streamflow ranks in the United States compared to mean quarterly streamflows for water years 1930–2021.[Some colors in the explanation may not be used on the map; **, for Puerto Rico and the Virgin Islands, 78 years of available data were used and the rank was adjusted accordingly]

Spring (April–June 2021) streamflow was ranked much below normal in Arizona, California, Colorado, Hawaii, Maine, Nevada, New Hampshire, New York, North Dakota, Oregon, Utah, Vermont, and Wyoming (fig. 7). Streamflow was ranked below normal in Connecticut, Idaho, Iowa, Massachusetts, Michigan, Minnesota, Montana, New Mexico, Pennsylvania, Puerto Rico and the Virgin Islands, Rhode Island, South Dakota, Washington, West Virginia, and Wisconsin. Streamflow was ranked above normal in Alabama, Arkansas, and Oklahoma. Streamflow was ranked much above normal in Louisiana, Mississippi, and Texas. Nationwide, spring-season streamflow ranked the 71st highest out of 92 years.

Figure 7. Spring (April–June 2021) statewide streamflow ranks in the United States compared to mean quarterly streamflows for water years 1930–2021.[Some colors in the explanation may not be used on the map; **, for Puerto Rico and the Virgin Islands, 78 years of available data were used and the rank was adjusted accordingly]

Summer (July–September 2021) streamflow was ranked much below normal in California, Idaho, Minnesota, North Dakota, Oregon, and Wyoming (fig. 8). Streamflow was ranked below normal in Colorado, Hawaii, Maine, Montana, Nevada, Puerto Rico and the Virgin Islands, South Dakota, Utah, and Washington. Streamflow was ranked above normal in Arkansas, the District of Columbia, Florida, Indiana, Kentucky, Michigan, Missouri, Ohio, Oklahoma, Texas, Vermont, and Wisconsin. Streamflow was ranked much above normal in Connecticut, Georgia, Illinois, Louisiana, Massachusetts, New Hampshire, New Jersey, New York, Pennsylvania, Rhode Island, and Tennessee. The highest streamflow was measured in Alabama and Mississippi. Nationwide, summer-season streamflow ranked the eighth highest out of 92 years

Figure 8. Summer (July–September 2021) statewide streamflow ranks in the United States compared to mean quarterly streamflows for water years 1930–2021.[Some colors in the explanation may not be used on the map; **, for Puerto Rico and the Virgin Islands, 78 years of available data were used and the rank was adjusted accordingly]

Assuming individual streamgage data are independent of other streamgage data and are normally distributed, the monthly streamflow is expected to be high (greater than the 95th percentile) at 5 percent of the streamgages and is expected to be low (less than the 5th percentile) at 5 percent of the streamgages during any given month. The percentages of streamgages reporting high streamflow in four months of water year 2021 (November 2020 and July, August, and September 2021) were greater than expected (5, 10, 6, and 8 percent, respectively; fig. 9). In contrast, there were eight months (October 2020 and March, April, May, June, July, August, and September 2021) with a greater-than-expected percentage of streamgages reporting low streamflow (5, 5, 9, 8, 7, 8, 5, and 5 percent, respectively).

Figure 9. Percentage of streamgages with high and low monthly streamflow, October 1999–September 2021.