Methods for Estimating Streamflow Statistics for Ungaged Streams in Maine


U.S. Geological Survey Fact Sheet 20004–3001 Version 1.01—ONLINE ONLY


by P.J. Lombard


This report is available as a pdf.


Streamflow statistics such as flood peak flows, monthly means and medians, and 7-day 10-year low flows (7Q10) are used for a variety of purposes, including the design of bridges, culverts, and flood-control structures; the management and regulation of flood plains and reservoirs; and the allocation of limited water resources during seasonal low flows and droughts.


Fig 1


Figure 1. Flood on the Kennebec River at Augusta, Maine, April 1987.


Although a number of rivers in Maine have long-term continuous gaging stations with up to 100 years of streamflow record, it often is necessary for hydrologists and water-resource managers to estimate flows on ungaged rivers, at ungaged locations on gaged rivers, or on gaged rivers with insufficient periods of record. To provide simple methods to estimate flow statistics at ungaged sites in Maine, the U.S. Geological Survey (USGS), in cooperation with State, local, and other Federal agencies, has developed methods that can be used by hydrologists, engineers, and managers to estimate characteristic flows based on measured hydrologic and climatic basin characteristics such as drainage area, wetland area, precipitation, elevation, and surficial geology.


Table 1. Methods for estimating streamflow statistics for ungaged basins by basin size.

  Basin size
Streamflow statistic
  0 to 1
square miles
1 to 10
square miles
square miles

Peak flow Under development Hodgkins (1999) Hodgkins (1999)
Annual mean/median Not available Not available Dudley (2004)
Monthly mean/median Not available Not available Dudley (2004)
Augustmedian—Aroostook County Lombard and others
Lombard and others
Dudley (2004)
August median—Downeast Under development Under development Dudley (2004)
7Q10—Statewide Not available Not available Dudley (2004)
Flow duration Not available Not available Not available


Peak Streamflows


Regression equations to estimate peak flows with 2- to 500-year recurrence intervals in basins with drainage areas from 1 to 1,650 square miles are presented in U.S. Geological Survey Water-Resources Investigations Report 99-4008 (Hodgkins, 1999). Guidelines provided in this report assist water resource managers in choosing the most appropriate equation to apply. Guideline criteria include evaluating the amount of development in the basin, the degree of regulation at the site of interest, the proximity of the site of interest to a gaging station, and the degree of accuracy compared to the ease of application. Statewide equations are derived to calculate the 2-, 5-, 10-, 50-, 100- and 500-year recurrence interval peak flows based on the drainage area of the basin and the percentage of wetlands in the basin. A simplified technique using only drainage area is quicker and easier to apply, but results in estimates of peak flows that are less accurate.


Estimating peak flows for small basins (less than 1 square mile) often is done with models such as the Rational method and the National Resources Conservation Service TR-20 model. A crest-stage gaging network currently is in operation on 15 small basins throughout Maine, which will allow the accuracy of the models to be evaluated.


Mid-Range Streamflows


The USGS recently developed improved statewide regression equations to estimate annual and monthly mean and median streamflows at ungaged streams with drainage areas from 10 to 1,420 square miles (Dudley, 2004). The new equations supersede those derived by the USGS 25 years ago (Parker, 1978). The new equations took advantage of 25 years of additional flow data and basin characteristics calculated with a Geographic Information System (GIS).


The statewide regression equations estimate annual and monthly mean and median flows using drainage basin characteristics including drainage area, surficial geology, precipitation, and distance from the coast.




Figure 2. 100-year drought at Piscataquis River at Dover-Foxcroft, Maine, August 2001.


Low Streamflows


A method to estimate the August median flow for ungaged basins in eastern Aroostook County is described in U.S. Geological Survey Water-Resources Investigations Report 03-4225 (Lombard and others, 2003). New generalized least-squares regression equations are applicable to small drainage basins from 1 to 40 square miles. Basin characteristics, including drainage area and mean elevation, are used to estimate August median flow. A similar study for small drainage basins in Washington and Hancock Counties currently is underway.


New regression equations to estimate the 7Q10 low-flow statistic and summer monthly means and medians on drainage basins >10 mi2. also have recently been developed (Dudley, 2004).


Future Directions


In U.S. Geological Survey Water-Resources Investigations Report 99-4008 (1999), Hodgkins indicated that very steep watersheds may behave differently than other watersheds with regards to peak flows. Work to investigate peak flows on steep watersheds has been proposed.




Dudley, R.W., 2004, Estimating monthly, annual, and low 7-day, 10-year streamflows for ungaged rivers in Maine: U.S. Geological Survey Scientific Investigations Report 2004-5026, 22 p.

Hodgkins, G.A., 1999, Estimating the magnitude of peak flows for streams in Maine for selected recurrence intervals: U.S. Geological Survey Water-Resources Investigations Report 99-4008, 45 p.

Lombard, P.J., Tasker, G. D., and Nielsen, M.G., 2003, August median streamflow on ungaged streams in eastern Aroostook County, Maine: U.S. Geological Survey Water-Resources Investigations Report, 03-4225, 20 p.

Parker, G.W., 1978, Methods for determining selected flow characteristics for streams in Maine: U.S. Geological Survey Open- File Report 78-871, 25 p.




Purpose and scope


Flood frequency estimates for gaged streamflow sites

Annual peak data

Historical peak discharges

Low-outlier thresholds

Skew coefficients

Documented extreme peak discharges

Potential extreme peak discharges


Selected references

Supplemental information

Table 1. Documented and potential extreme peak discharges and flood frequency estimates for selected streamflow-gaging stations with at least 8 years of annual peak-discharge data from unregulated, regulated, and urban basins within and near Oklahoma

Table 2. Documented and potential extreme peak discharges for selected indirect measurement sites without streamflow-gaging stations and streamflow-gaging stations with short periods in basins within Oklahoma .

For additional information write to:


District Chief

U.S. Geological Survey

196 Whitten Road

Augusta, ME 04330



For more information about the USGS and its products:

Telephone: 1-888-ASK-USGS World Wide Web:


The full report (version 1.01)is available in pdf format and is 1.1 MB in size.


Version History


If you have Adobe® Acrobat® or Adobe® Acrobat® Reader® installed on your computer, you may view and print the PDF version of this report. Acrobat Reader, is a free download it from Adobe Systems, Inc.  Users with disabilities can view information concerning accessibility at


Return to Top


U.S. Department of the Interior, U.S. Geological Survey
Persistent URL:
Page Contact Information: Contact USGS
Last modified: Tuesday, November 29 2016, 05:59:14 PM
FirstGov button  Take Pride in America button