Annual runoff in the Nation’s rivers and streams during water year 2020 (11.10 inches) was higher than the long-term (1930–2020) mean annual runoff of 9.40 inches for the contiguous United States (fig. 1). Nationwide, the 2020 annual average streamflow ranked the 10th highest out of the 91 years.

Figure 1. Annual runoff in the contiguous United States, water years 1930–2020.

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 below normal in California, Colorado, Hawaii, Nevada, and Oregon (fig. 2). Streamflow was above normal in the District of Columbia, Georgia, Iowa, Ohio, Oklahoma, South Carolina, Virginia, and West Virginia. Streamflow was much above normal in Alabama, Arkansas, Illinois, Kentucky, Minnesota, Mississippi, Missouri, Nebraska, North Carolina, North Dakota, South Dakota, Tennessee, and Wisconsin. Record maximum streamflow was measured in Michigan.

Figure 2. Statewide streamflow ranks of the United States for water year 2020 compared to mean annual streamflows for water years 1930–2020.[Some colors in the explanation may not be used on the map; **, for Puerto Rico combined with the Virgin Islands, 77 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, watershed-based framework for depicting streamflow variations

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

Annual mean streamflow was ranked below normal levels in the California, Hawaii, Great Basin, Rio Grande, and Upper Colorado regions (fig. 4). Annual mean streamflow was ranked above normal in the Lower Mississippi, Missouri, and South Atlantic-Gulf regions. Streamflow was ranked much above normal in the Arkansas-White-Red, Great Lakes, Ohio, Souris-Red-Rainy, and Upper Mississippi regions. Record maximum streamflow was measured in the Tennessee region.

Figure 4. Regional streamflow ranks in the United States for water year 2020 compared to mean annual streamflows for water years 1930–2020.[Some colors in the explanation may not be used on the map; **, for Puerto Rico combined with the Virgin Islands, 77 years of available data were used, and the rank was adjusted accordingly]

Autumn (October–December 2019) streamflow was ranked much below normal in Oregon (fig. 5). Streamflow was ranked below normal in Hawaii, Puerto Rico combined with the Virgin Islands, and Washington. Streamflow was ranked above normal in Alabama, Arkansas, Kansas, Kentucky, Maine, Mississippi, Missouri, Montana, New Mexico, New York, Oklahoma, Vermont, and Wyoming. Streamflow was ranked much above normal in Illinois, Iowa, and Nebraska. Record maximum streamflow was measured in Michigan, Minnesota, North Dakota, South Dakota, and Wisconsin. Nationwide, autumn-season streamflow ranked the highest out of 91 years.

Figure 5. Autumn (October–December 2019) statewide streamflow ranks in the United States compared to mean quarterly streamflows for water years 1930–2020.[Some colors in the explanation may not be used on the map; **, for Puerto Rico combined with the Virgin Islands, 77 years of available data were used, and the rank was adjusted accordingly]

Winter (January–March 2020) streamflow was ranked much below normal in California (fig. 6). Streamflow was ranked below normal in Delaware, Idaho, and Rhode Island. Streamflow was ranked above normal in Iowa, North Carolina, North Dakota, Ohio, Vermont, Washington, and Wyoming. Streamflow was ranked much above normal in Alabama, Arkansas, Georgia, Illinois, Indiana, Kansas, Kentucky, Michigan, Minnesota, Mississippi, Missouri, Nebraska, Oklahoma, Puerto Rico and the Virgin Islands, South Carolina, South Dakota, Tennessee, and Wisconsin. Nationwide, winter-season streamflow ranked the sixth highest out of 91 years.

Figure 6. Winter (January–March 2020) statewide streamflow ranks in the United States compared to mean quarterly streamflows for water years 1930–2020. [[Some colors in the explanation may not be used on the map; **, for Puerto Rico combined with the Virgin Islands, 77 years of available data were used, and the rank was adjusted accordingly]

Spring (April–June 2020) streamflow was ranked much below normal in Hawaii (fig. 7). Streamflow was ranked below normal in Colorado, Nevada, New Mexico, New York, Oregon, Puerto Rico combined with the Virgin Islands, and Vermont. Streamflow was ranked above normal in Arkansas, the District of Columbia, Kentucky, Michigan, Missouri, North Dakota, South Carolina, South Dakota, Tennessee, and Wisconsin. Streamflow was ranked much above normal in North Carolina, Virginia, and West Virginia. Nationwide, spring-season streamflow ranked the 34th highest out of 91 years.

Figure 7. Spring (April–June 2020) statewide streamflow ranks in the United States compared to mean quarterly streamflows for water years 1930–2020.[Some colors in the explanation may not be used on the map; **, for Puerto Rico combined with the Virgin Islands, 77 years of available data were used, and the rank was adjusted accordingly]

Summer (July–September 2020) record minimum streamflow was measured in Arizona and Maine (fig. 8). Streamflow was ranked much below normal in Colorado, Connecticut, Hawaii, New Mexico, Rhode Island, and Vermont. Streamflow was ranked below normal in Massachusetts, New Hampshire, and Utah. Streamflow was ranked above normal in Alabama, Illinois, Kentucky, Louisiana, Maryland, Mississippi, North Carolina, Oklahoma, South Carolina, South Dakota, Tennessee, and Virginia. Streamflow was ranked much above normal in Arkansas, Delaware, the District of Columbia, Michigan, North Dakota, and Wisconsin. Nationwide, summer-season streamflow ranked the 25th highest out of 91 years.

Figure 8. Summer (July–September 2020) statewide streamflow ranks in the United States compared to mean annual streamflows for water years 1930–2020[Some colors in the explanation may not be used on the map; **, for Puerto Rico combined with the Virgin Islands, 77 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 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 six months of water year 2020 (October, November, and December 2019 and January, February, and May 2020) were greater than expected (10, 8, 7, 11, 10, and 7 percent, respectively; fig. 9). In contrast, there was only one month (September 2020) with a greater-than-expected percentage of streamgages reporting low flow (6 percent).

Figure 9. Percentage of streamgages with high and low monthly streamflow, October 1999– September 2020.