Annual runoff in the Nation’s rivers and streams during water year 2018 (9.82 inches; fig. 1) was higher than the long-term (water years 1930–2018) mean annual runoff of 9.33 inches. Nationwide, the 2018 streamflow ranked 33d highest out of the 89 years.

Figure 1. Annual runoff in the United States, water years 1930–2018.

Record low streamflow levels were reported in Arizona (fig. 2). Streamflow was much below normal in Colorado and New Mexico. Streamflow was below normal in Utah, Alaska, Kansas, and Oregon. Streamflow was above normal in Delaware, North Carolina, Nebraska, Wyoming, Tennessee, Indiana, Minnesota, District of Columbia, Kentucky, and South Dakota. Streamflow was much above normal in Iowa, Michigan, Maryland, Wisconsin, West Virginia, Ohio, Montana, and Pennsylvania. Half of the States had streamflow in the normal range.

Figure 2. Statewide streamflow ranks of the United States for water year 2018 relative to mean annual streamflow for water years 1930–2018. [**For Puerto Rico and the Virgin Islands, 75 years of available data were used and the groupings of ranks were adjusted accordingly.]

The United States, Puerto Rico, and the Virgin Islands are divided into 21 large drainages, or water-resources regions (fig. 3). These waterresources 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.

much below normal levels in the Lower Colorado region (fig. 4). Below normal streamflow was reported in the Upper Colorado, Rio Grande, Alaska, and California regions. Streamflow was above normal in the Tennessee, Upper Mississippi, Great Lakes, and Mid-Atlantic regions. Much above normal streamflow was reported in the Ohio region.

Figure 4.Regional streamflow ranks in the United States for water year 2018 relative to mean annual streamflow for water years 1930–2018. [**For Puerto Rico and the Virgin Islands, 75 years of available data were used and the groupings of ranks were adjusted accordingly.]

For the autumn season (October–December 2017), streamflow was much below normal in Arizona (fig. 5). Streamflow was below normal in New Jersey, Virginia, Arkansas, Louisiana, South Carolina, District of Columbia, Maryland, North Carolina, and Texas. Above normal streamflow was reported in Montana, Idaho, Wisconsin, New Mexico, and Florida. Much above normal streamflow was reported in Minnesota, South Dakota, Michigan, Wyoming, Puerto Rico and the Virgin Islands, and Nebraska. Streamflows were reported at record high levels in Nevada. Nationwide, autumn-season streamflow ranked 53d highest out of 89 years.

Figure 5.Autumn (October–December 2017) statewide ranks in the United States relative to mean annual streamflow for water years 1930–2018.[**For Puerto Rico and the Virgin Islands, 75 years of available data were used and the groupings of ranks were adjusted accordingly.]

For the winter season (January–March 2018), streamflow was at a record low in Arizona (fig. 6). Streamflow was below normal in South Carolina, Georgia, New Mexico, District of Columbia, Florida, Alaska, and North Carolina. Above normal streamflow was reported in Michigan, Connecticut, Tennessee, Vermont, South Dakota, West Virginia, Indiana, Arkansas, Maine, Kentucky, Idaho, New York, Pennsylvania, New Hampshire, Massachusetts, Puerto Rico and the Virgin Islands, Ohio, and Wyoming. Streamflow was much above normal in Montana, Rhode Island, and Nevada. Nationwide, winter-season streamflow ranked 37th highest out of 89 years.

Figure 6. Winter (January–March 2018) statewide ranks in the United States relative to mean annual streamflow for water years 1930–2018. [**For Puerto Rico and the Virgin Islands, 75 years of available data were used and the groupings of ranks were adjusted accordingly.]

For the spring season (April–June 2018), streamflow was much below normal in Arizona, New Mexico, Colorado, Utah, and Kansas (fig. 7). Below normal streamflow was reported in Texas, Oklahoma, and New Hampshire. Above normal streamflow was reported in Florida, New Jersey, Iowa, Nevada, South Dakota, Wisconsin, Hawaii, Tennessee, Pennsylvania, North Carolina, Wyoming, and Ohio. Streamflow was much above normal in Delaware, District of Columbia, Virginia, Maryland, West Virginia, and Montana. Nationwide, spring-season streamflow ranked 34th highest out of 89 years.

Figure 7. Spring (April–June 2018) statewide ranks in the United States relative to mean annual streamflow for water years 1930–2018. [**For Puerto Rico and the Virgin Islands, 75 years of available data were used and the groupings of ranks were adjusted accordingly.]

For the summer season (July–September 2018), streamflow was much below normal in Colorado, New Mexico, Arizona, Maine, and Alaska (fig. 8). Below normal streamflow was reported in Washington, Utah, and Oregon. Above normal streamflow was observed in Michigan, South Carolina, Oklahoma, Tennessee, New Hampshire, New York, Illinois, Hawaii, Ohio, New Jersey, Delaware, and Indiana. Much above normal streamflow was reported in Nebraska, Wisconsin, Massachusetts, Minnesota, Virginia, Connecticut, Kentucky, South Dakota, Iowa, and West Virginia. Record high streamflow was reported in North Carolina, Pennsylvania, District of Columbia, and Maryland. Nationwide, summer-season streamflow ranked 3d highest out of 89 years.

Figure 8. Summer (July–September 2018) statewide ranks in the United States relative to mean annual streamflow for water years 1930–2018. [**For Puerto Rico and the Virgin Islands, 75 years of available data were used and the groupings of ranks were adjusted accordingly.]

Assuming that individual streamgages act independently of each other, it is expected that the mean streamflow at 5 percent of the streamgages will be high (greater than the 95th percentile) and 5 percent will be low (less than the 5th percentile) in any given month. The percentages of streamgages reporting high streamflow in 5 months of water year 2018 (October 2017 and February, May, August, and September 2018) were higher than expected (6, 11, 9, 9, and 18 percent, respectively; fig. 9). In contrast, there was only one month (June 2018) with a greater-than-expected percentage of streamgages with low flows (6 percent).

Figure 9. Percentage of streamgages with monthly streamflow greater than the 95th percentile (in blue) or less than the 5th percentile (in red), October 1999–September 2018.