This U.S. Geological Survey summary ranks streamflow in water year 2021 against the 92 water years 1930–2021. A water year runs from October 1 to September 30 and is named for the year it ends in: water year 2021 ran from October 1, 2020, to September 30, 2021. Years before 1930 are left out because there were too few streamgages then for meaningful statistics in most regions.
How to read the ranks
- Runoff is the water flowing through a stream divided by its drainage area — an equivalent depth of water over the basin — so that areas of different size can be compared. It is computed from all streamgages with complete records in an area.
- Rank 1 is the highest annual flow of the 92 years; rank 92 the lowest.
| Category | Percentile of flow |
|---|---|
| Much above normal | above 90 |
| Above normal | 76–90 |
| Normal | 25–75 |
| Below normal | 10–24 |
| Much below normal | below 10 |
Some of the data are provisional and may change. "States" here include the District of Columbia, and Puerto Rico combined with the Virgin Islands.
The nation: an average year
Annual runoff in the contiguous United States was 9.43 inches — almost exactly the 1930–2021 mean of 9.42 inches. Nationwide, annual average streamflow ranked 46th highest of 92.
Figure 1. Annual runoff in the contiguous United States, water years 1930–2021. USGS.
By state
| Category | States |
|---|---|
| Much below normal | California, Colorado, Idaho, Nevada, New Mexico, North Dakota, Utah, Wyoming |
| Below normal | Alaska, Arizona, Hawaii, Maine, Minnesota, Oregon, Vermont |
| Above normal | Alabama, Arkansas, Connecticut, District of Columbia, Florida, Kentucky, Maryland, Mississippi, New Jersey, Tennessee |
| Much above normal | Delaware, Louisiana, North Carolina, Virginia |
Figure 2. Statewide streamflow ranks, water year 2021, against 1930–2021. USGS.
By region
The United States, D.C., and Puerto Rico with the Virgin Islands fall into 21 water-resources regions — large drainages defined by surface topography: the basin of one major river, the combined basins of several rivers (the Texas-Gulf region, for one, drains several rivers into the Gulf of Mexico), or an island or island group.
Figure 3. Water-resources regions of the United States. USGS.
| Category | Regions |
|---|---|
| Lowest of all | Rio Grande |
| Much below normal | California, Great Basin, Upper Colorado |
| Below normal | Alaska, Hawaii, Lower Colorado, Pacific Northwest, Souris-Red-Rainy |
| Above normal | Arkansas-White-Red, Lower Mississippi, South Atlantic-Gulf, Tennessee, Texas-Gulf |
Figure 4. Regional streamflow ranks, water year 2021, against 1930–2021. USGS.
Season by season
| Season | National rank (of 92) | Highest | Much above normal | Above normal | Below normal | Much below normal | Lowest |
|---|---|---|---|---|---|---|---|
| Autumn (Oct–Dec 2020) | 33rd | — | Delaware, D.C., Louisiana, Maryland, North Carolina, Virginia | Florida, Maine, Mississippi, New Jersey, South Carolina, South Dakota, Wisconsin | California | Colorado, Hawaii, New Mexico, Utah | Arizona |
| Winter (Jan–Mar 2021) | 47th | — | North Carolina | Delaware, Kansas, Kentucky, Missouri, Nebraska, South Carolina, Wisconsin, Virginia | Idaho, New York, North Dakota, Pennsylvania, Puerto Rico and Virgin Islands, Rhode Island | California, Colorado, Nevada, New Mexico, Utah | Arizona |
| Spring (Apr–Jun 2021) | 71st | — | Louisiana, Mississippi, Texas | Alabama, Arkansas, Oklahoma | Connecticut, Idaho, Iowa, Massachusetts, Michigan, Minnesota, Montana, New Mexico, Pennsylvania, Puerto Rico and Virgin Islands, Rhode Island, South Dakota, Washington, West Virginia, Wisconsin | Arizona, California, Colorado, Hawaii, Maine, Nevada, New Hampshire, New York, North Dakota, Oregon, Utah, Vermont, Wyoming | — |
| Summer (Jul–Sep 2021) | 8th | Alabama, Mississippi | Connecticut, Georgia, Illinois, Louisiana, Massachusetts, New Hampshire, New Jersey, New York, Pennsylvania, Rhode Island, Tennessee | Arkansas, D.C., Florida, Indiana, Kentucky, Michigan, Missouri, Ohio, Oklahoma, Texas, Vermont, Wisconsin | Colorado, Hawaii, Maine, Montana, Nevada, Puerto Rico and Virgin Islands, South Dakota, Utah, Washington | California, Idaho, Minnesota, North Dakota, Oregon, Wyoming | — |
Nationally, spring streamflow ranked only 71st of 92, and summer 8th of 92.
Figure 5. Autumn (October–December 2020). USGS.
Figure 6. Winter (January–March 2021). USGS.
Figure 7. Spring (April–June 2021). USGS.
Figure 8. Summer (July–September 2021). USGS.
High and low months
If each streamgage behaved independently and flows were normally distributed, in any month about 5% of gages would report high flow (above the 95th percentile) and 5% low flow (below the 5th). In water year 2021, the reported shares met or exceeded that in:
| Months | Share of streamgages | |
|---|---|---|
| High flow | 4 — November 2020; July, August, September 2021 | 5%, 10%, 6%, 8% |
| Low flow | 8 — October 2020; March, April, May, June, July, August, September 2021 | 5%, 5%, 9%, 8%, 7%, 8%, 5%, 5% |
Figure 9. Percentage of streamgages with high (above the 95th percentile) and low (below the 5th) monthly streamflow, October 1999–September 2021. USGS.
The network
In water year 2021 the USGS ran more than 8,400 year-round, real-time streamgages. Their data serve water management, flood and drought monitoring, bridge design and recreation.
- Real-time data: WaterWatch.
- Real-time and archived data: the USGS National Water Information System.
- The National Streamflow Network, part of the Groundwater and Streamflow Information Program, is run mainly by the USGS, and funded by the USGS and about 1,885 federal, state, tribal, regional and local partners. More: USGS streamflow monitoring.
Sources
Based on "Streamflow—Water Year 2021," U.S. Geological Survey fact sheet; a work of the United States government in the public domain. The source lists Delaware, Louisiana, North Carolina and Virginia as "much normal"; its map (Figure 2) shows them much above normal, as given here.
Licence: CC0 1.0 (public domain) · Adapted from pubs.er.usgs.gov
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