This summary is published as USGS Fact Sheet 2012-3085 and can be downloaded as a PDF file

Streamflow of 2011 - Water Year Summary

U.S. Geological Survey
Reston, Virginia

January 2012

National Overview
Regional Patterns
Seasonal Characteristics
High and Low Flows

Introduction

The maps and graphs in this summary describe streamflow conditions for water-year 2011 (October 1, 2010 to September 30, 2011) in the context of the 82-year period 1930-2011, unless otherwise noted. The illustrations are based on observed data from the U.S. Geological Survey's (USGS) National Streamflow Information Program (http://water.usgs.gov/nsip). The period 1930-2011 was used because prior to 1930, the number of streamgages was too small to provide representative data for computing statistics for most regions of the country.

In the summary, reference is made to the term "runoff," which is the depth to which a river basin, State, or other geographic area would be covered with water if all the streamflow within the area during a specified time period was uniformly distributed upon it. Runoff quantifies the magnitude of water flowing through the Nation's rivers and streams in measurement units that can be compared from one area to another.

Each of the maps and graphs below can be expanded to a larger view by clicking on the image. In all the graphics, a rank of 1 indicates the highest flow of all years analyzed.

National Overview

Average runoff in the Nation’s rivers and streams during 2011 (10.50 inches) was higher than the long-term annual mean for the United States (9.31 inches). Nationwide, 2011 streamflow ranked 16^th out of the 82 years in the period 1930-2011. Note that in previous water-year summaries the median runoff, not the average runoff, was compared among time periods.

Streamflow was at record high levels (ranking 1^st in 82 years) in the states of Montana, North Dakota, South Dakota, Minnesota, New York, and Vermont. Above normal and much-above normal streamflow characterized the remaining states in the Northeast, North, West, and Puerto Rico. In contrast, streamflow was at record low levels (ranking 82^nd in 82 years) in Texas and Alaska. Below normal and much-below normal streamflow was prevalent cross the Southeast and South. Only 11 states had streamflow in the normal range.

* For Puerto Rico, 68 years of available data were used.

Regional Patterns

The United States (including Puerto Rico) is divided into 21 large drainages, or water resources regions. These hydrologic areas are based on surface topography and contain either the drainage area of a major river, such as the Columbia, the combined drainage areas of a series of rivers, such as the Texas-Gulf region which includes a number of 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.

Streamflow was at record high levels (ranking 1st in 82 years) in the water resource regions of the Souris-Red-Rainy and Mid Atlantic regions. Above normal and much-above normal streamflow occurred in all northeastern, northern, and western regions, and in the Caribbean. In contrast, streamflow was at record low levels (ranking 82nd in 82 years) in the South Atlantic-Gulf, Texas-Gulf, and Alaska regions, respectively. Below-normal and much-below normal were reported in the Lower Mississippi, and Rio Grande regions.

Seasonal Characteristics

		Autumn season (October-December) streamflow was at record high levels (ranking 1^st in 82 years) in the states of North Dakota, South Dakota, and Minnesota. Above and much-above normal flows were reported in the northeastern, northern, and southwestern states. Florida had record low flow (ranking 82^nd in 82 years). Most of the other southeastern and southern states , as well as Alaska had below and much below normal streamflows. Nationwide, autumn season streamflow ranked 44^th out of 82 years. * For Puerto Rico, 68 years of available data were used.
		Winter season (January-March) streamflow was above or much-above normal in many northern and northeastern states. Record low flow (ranking 82^nd in 82 years) occurred in Alaska. Below normal and much-below normal streamflow occurred in most southern and southeastern states. Nationwide, winter season streamflow ranked 31^st out of 82 years. *For Puerto Rico, 68 years of available data were used.
		Spring season (April-June) streamflow was at record high levels (ranking 1st in 82 years) in the states of Montana, North Dakota, South Dakota, Illinois, Indiana, Ohio, Kentucky, and Vermont. Streamflow was above or much-above normal in the northern and western states, Missouri, Arkansas, Tennessee, Hawaii, and Puerto Rico. In contrast, southeastern and southern states reported below or much-below normal flows. Streamflow in Texas was at record low levels. On a nationwide basis, spring season streamflow was much-above normal, ranking 5^th in 82 years. * For Puerto Rico, 68 years of available data were used.
		Summer season (July-September) streamflow was at record high levels (ranking 1st in 82 years) in the states of Montana, North Dakota, Idaho, New Jersey, New York, and Vermont. This was the third season out of four that North Dakota experienced record high levels. Streamflow was above or much-above normal in the northeastern, northern, northwestern and western states, and Puerto Rico. In contrast, southeastern and southern states and Alaska reported below or much-below normal flows. Streamflow in Georgia was at record low levels. On a nationwide basis, summer season streamflow was much-above normal, ranking 13^th in 82 years. * For Puerto Rico, 68 years of available data were used.

High and Low Flows

In any given month, on average, it is expected that five percent of the streamgages will experience very high (>95^th percentile) and very low (<5^th percentile) average streamflow. During water year 2011, nine months (October, December, March, April, May, June, July, August, and September) had a greater than expected percentage of streamgages reporting very high streamflow (6, 6, 12,16, 15,11, 14, 14, and 15 percent, respectively). In contrast, there were 5 months (May, June, July, August, and September) with a greater than expected percentage of streamgages with very low flows (6, 9, 7, 7, and 6 percent, respectively).

The bankfull streamflow is defined as the highest daily mean streamflow value expected to occur, on average, once in every 2.3 years. In 2011, 50 percent of streamgages had a daily mean streamflow value above the bankfull level. This value is greater than the expected number (43 percent) to occur in any given year. Since 1950, the largest number of streamgages reporting higher than bankfull streamflow in any one year was 68 percent, which occurred in 1996.

percentage above 10th percentile of annual 7-day minimum streamflow

The 10^th percentile 7-day low flow is defined as the lowest 7-day average streamflow expected to occur, on average, once in every 10 years. In water-year 2011, 17 percent of the streamgages reported a 7-day low flow less than the 10^th percentile 7-day low flow. The expected number to occur in any given year is 10 percent. Since 1950, the largest percentage of streamgages reporting a 7-day low flow less than the 10th percentile 7-day low flow was 25 percent, which occurred in 1954.

Additional Information

The USGS operates a network of nearly 10,000 streamgages nationwide, many in real-time. Current information derived from these stations is available on the web at http://waterwatch.usgs.gov. Tables of data that summarize historical streamflow conditions by State, beginning in the year 1900, can be accessed at http://waterwatch.usgs.gov/?m=statesum. These tables are updated every few months to reflect the most current streamflow data.

The streamflow information used to prepare this summary is also used for water management, monitoring floods and droughts, bridge design, and for many recreational activities. To obtain real-time and archived streamflow data and information, visit http://water.usgs.gov/nwis. Although the national streamgage network is operated primarily by the USGS, it is funded by a partnership of 850 agencies at the Federal, State, Tribal, and local levels. For more information about the streamgage network, see http://water.usgs.gov/nsip/.

For additional information, contact:
	Harry Lins U.S. Geological Survey 415 National Center Reston, VA 20192 Email: hlins@usgs.gov